diff --git a/tungston/core/altitude.py b/tungston/core/altitude.py
index c40f67b..ce26fb9 100644
--- a/tungston/core/altitude.py
+++ b/tungston/core/altitude.py
@@ -67,8 +67,20 @@ def span(*altitudes):
     if not altitudes: return ANYWHERE
     result = Altitude("", ANYWHERE.top, ANYWHERE.bottom)
 
-    for altitude in altitudes:
-        result.bottom = min(result.bottom, altitude.bottom)
-        result.top = max(result.top, altitude.top)
+    bottom = OVERGROUND[-1].top
+    bottomAltitude = None
 
+    top = UNDERGROUND[-1].bottom
+    topAltitude = None
+
+    for altitude in altitudes:
+        if altitude.bottom < bottom:
+            bottom = altitude.bottom
+            bottomAltitude = altitude
+
+        if altitude.top > top:
+            top = altitude.top
+            topAltitude = altitude
+
+    result = Altitude(f"{bottomAltitude.name}-{topAltitude.name}", bottom, top)
     return result
diff --git a/tungston/core/altitude_test.py b/tungston/core/altitude_test.py
index 27edd03..48fd2b6 100644
--- a/tungston/core/altitude_test.py
+++ b/tungston/core/altitude_test.py
@@ -13,6 +13,6 @@ def mergeRange():
 # Tests ############################################################################################
 
 def test_span(merged):
+    assert merged.name == "lowlands-uplands"
     assert merged.bottom == A.LOWLANDS.bottom
     assert merged.top == A.UPLANDS.top
-
diff --git a/tungston/core/ecology/biomefilter.py b/tungston/core/ecology/biomefilter.py
index a01e517..f02a0ea 100644
--- a/tungston/core/ecology/biomefilter.py
+++ b/tungston/core/ecology/biomefilter.py
@@ -31,6 +31,30 @@ class BiomeFilter:
         self.required = required or []
         self.prohibited = prohibited or []
 
+    def __str__(self) -> str:
+        required = []
+        for trait in self.required:
+            if isinstance(trait, BiomeTrait):
+                required.append(str(trait))
+            else:
+                required.append("(" +
+                    ", ".join([str(option) for option in trait]) +
+                ")")
+
+        prohibited = []
+        for trait in self.prohibited:
+            prohibited.append(str(trait))
+
+        result = "".join([
+            "must:[",
+            ", ".join(required),
+            "], not:[",
+            ", ".join(prohibited),
+            "]"
+        ])
+
+        return result
+
     def __repr__(self) -> str:
         return f"BiomeFilter([{repr(self.required)}], [{repr(self.prohibited)}])"
 
diff --git a/tungston/core/fauna/group.py b/tungston/core/fauna/group.py
index 24c4145..5001af3 100644
--- a/tungston/core/fauna/group.py
+++ b/tungston/core/fauna/group.py
@@ -5,7 +5,7 @@ class Group:
     How many individuals are commonly present when encountering a creature.
     """
 
-    def __init__(self, name, smallest, largest):
+    def __init__(self, name:str, smallest:int, largest:int):
         self.name = name
         self.smallest = smallest
         self.largest = largest
@@ -42,9 +42,19 @@ ALL = [SOLO, PAIR, FAMILY, TROUP, HERD]
 def merge(*groups):
     if not groups: return SOLO
 
-    result = Group(f"", HERD.largest, SOLO.smallest)
-    for group in groups:
-        result.smallest = min(result.smallest, group.smallest)
-        result.largest = max(result.largest, group.largest)
+    smallest = ALL[-1].largest
+    smallestGroup = None
 
-    return result
+    largest = ALL[0].smallest
+    largestGroup = None
+
+    for group in groups:
+        if group.smallest < smallest:
+            smallest = group.smallest
+            smallestGroup = group
+
+        if group.largest > largest:
+            largest = group.largest
+            largestGroup = group
+
+    return Group(f"{smallestGroup.name}-{largestGroup.name}", smallest, largest)
diff --git a/tungston/core/fauna/mobcatalog.py b/tungston/core/fauna/mobcatalog.py
index 232dd05..fcda846 100644
--- a/tungston/core/fauna/mobcatalog.py
+++ b/tungston/core/fauna/mobcatalog.py
@@ -1,16 +1,26 @@
-from tungston.core.fauna.mob import Mob
+from collections.abc import Iterator
+from tungston.core.fauna.mobplacement import MobPlacement
 
 
 # Class ############################################################################################
 
 class MobCatalog:
 
-    def __init__(self, mobs):
-        self.mobs = mobs
+    def __init__(self, mobs:tuple[MobPlacement]):
+        self._mobs = {}
 
-    def findByName(self, name):
-        for mob in self.mobs:
-            if mob.name == name:
-                return mob
+        for mob in mobs:
+            self.add(mob)
 
-        return None
+    def add(self, placement:MobPlacement) -> "MobCatalog":
+        if placement.mob.name in self._mobs:
+            raise Exception("Catalog already has an entry for {mob.name}")
+
+        self._mobs[placement.mob.name] = placement
+
+    def all(self) -> Iterator[MobPlacement]:
+        for placement in self._mobs.values():
+            yield placement
+
+    def get(self, name:str) -> MobPlacement:
+        return self._mobs.get(name, None)
diff --git a/tungston/core/fauna/mobplacement_test.py b/tungston/core/fauna/mobplacement_test.py
index 1f6701f..66a2eed 100644
--- a/tungston/core/fauna/mobplacement_test.py
+++ b/tungston/core/fauna/mobplacement_test.py
@@ -30,7 +30,7 @@ def test_cowsInFields(placement):
         "Placement{" +
             "habitats: [" +
                 "Habitat{" +
-                    "altitude: [anywhere <-64 to 320>], " +
+                    "altitude: anywhere <-64 to 320>, " +
                     "biomeFilter: BiomeFilter([[Flora<field>, Heat<temperate>]], [[]]), " +
                     "seasons: [Season<spring>, Season<summer>, Season<autumn>, Season<winter>], " +
                     "group: solo<1 to 1>, " +
diff --git a/tungston/core/geology/deposit/metaldeposit_test.py b/tungston/core/geology/deposit/metaldeposit_test.py
index af39fd9..a171427 100644
--- a/tungston/core/geology/deposit/metaldeposit_test.py
+++ b/tungston/core/geology/deposit/metaldeposit_test.py
@@ -44,7 +44,7 @@ def test_repr(bifIron):
     assert repr(bifIron) == (
         "MetalDeposit<bifiron>{" +
             "scarcity: Scarcity<common>, " +
-            "biomeFilter: BiomeFilter([[Flora<forest>]], [[]]), " +
+            "biomeFilter: must:[Flora: forest], not:[], " +
             "inclusions: [" +
                 "Inclusion{mineral: iron, weight: 60}, " +
                 "Inclusion{mineral: quartz, weight: 40}" +
diff --git a/tungston/core/habitat.py b/tungston/core/habitat.py
index d2ec6ce..dbe4e8f 100644
--- a/tungston/core/habitat.py
+++ b/tungston/core/habitat.py
@@ -22,7 +22,7 @@ class Habitat:
     def __init__(
         self,
 
-        altitudes:tuple[Altitude]|Altitude=[A.ANYWHERE],
+        altitude:Altitude=A.ANYWHERE,
         biomeFilter:BiomeFilter=None,
         seasons:tuple[Season]=E.ALL,
 
@@ -30,7 +30,7 @@ class Habitat:
         location:Location=L.OUTSIDE,
         scarcity:Scarcity=C.SPARSE,
     ):
-        self.altitudes = (altitudes,) if isinstance(altitudes, Altitude) else altitudes
+        self.altitude = altitude
         self.biomeFilter = biomeFilter or BiomeFilter()
         self.seasons = (seasons,) if isinstance(seasons, Season) else seasons
 
@@ -41,19 +41,19 @@ class Habitat:
         self._source = None
 
     def __str__(self) -> str:
-        return "".join(str(p) for p in [
-            self.altitude, "|",
+        return "".join([str(p) for p in [
+            ", ".join(self.altitude), "|",
             self.biomeFilter, "|",
-            self.seasons, "|",
+            ", ".join([str(s) for s in self.seasons]), "|",
             self.group, "|",
             self.location, "|",
             self.scarcity
-        ])
+        ]])
 
     def __repr__(self) -> str:
         return "".join([str(p) for p in [
             "Habitat{",
-                "altitude: [", ", ".join([repr(a) for a in self.altitudes]), "], ",
+                "altitude: ", repr(self.altitude), ", ",
                 "biomeFilter: ", repr(self.biomeFilter), ", ",
                 "seasons: [", ", ".join([repr(s) for s in self.seasons]), "], ",
                 "group: ", repr(self.group), ", ",
@@ -64,7 +64,7 @@ class Habitat:
 
     def derive(self, **kwargs):
         kwargs = {
-            "altitudes": self.altitudes,
+            "altitude": self.altitude,
             "biomeFilter": self.biomeFilter,
             "seasons": self.seasons,
             "group": self.group,
diff --git a/tungston/core/habitat_test.py b/tungston/core/habitat_test.py
index 287df29..95cd64e 100644
--- a/tungston/core/habitat_test.py
+++ b/tungston/core/habitat_test.py
@@ -35,7 +35,7 @@ def test_derive(derivedHabitats):
 def test_repr(fields):
     assert repr(fields) == (
         "Habitat{" +
-            "altitude: [anywhere <-64 to 320>], " +
+            "altitude: anywhere <-64 to 320>, " +
             "biomeFilter: BiomeFilter([[Flora<field>, Heat<temperate>]], [[]]), " +
             "seasons: [Season<spring>, Season<summer>, Season<autumn>, Season<winter>], " +
             "group: solo<1 to 1>, " +
diff --git a/tungston/core/placement_test.py b/tungston/core/placement_test.py
index 73774c1..054b4e5 100644
--- a/tungston/core/placement_test.py
+++ b/tungston/core/placement_test.py
@@ -25,7 +25,7 @@ def test_repr(placement):
         "Placement{" +
             "habitats: [" +
                 "Habitat{" +
-                    "altitude: [anywhere <-64 to 320>], " +
+                    "altitude: anywhere <-64 to 320>, " +
                     "biomeFilter: BiomeFilter([[Flora<field>, Heat<temperate>]], [[]]), " +
                     "seasons: [Season<spring>, Season<summer>, Season<autumn>, Season<winter>], " +
                     "group: solo<1 to 1>, " +
diff --git a/tungston/generator/markdown/mobreport.py b/tungston/generator/markdown/mobreport.py
new file mode 100644
index 0000000..49d5532
--- /dev/null
+++ b/tungston/generator/markdown/mobreport.py
@@ -0,0 +1,49 @@
+from tungston.core.world import World
+from tungston.core.ecology.biometrait import BiomeTrait
+from tungston.generator.markdown.report import Report
+
+
+# Class ############################################################################################
+
+class MobReport(Report):
+
+    def __init__(self, world:World):
+        super().__init__("mobs.md", world)
+
+    def build(self):
+        for placement in self.world.mobs.all():
+            self.line(f"# Mob: {placement.mob.name} <{placement.mob.gameId}>")
+            self.line()
+
+            self.indent()
+            for index, habitat in enumerate(placement.habitats, start=1):
+                self.text("* habitat ").line(index)
+                self.indent()
+                self.text("* altitude: ").line(habitat.altitude)
+                self.line("* biomeFilter:")
+
+                self.indent()
+                for trait in habitat.biomeFilter.required:
+                    if isinstance(trait, BiomeTrait):
+                        self.text("* ").line(trait)
+                    else:
+                        category = type(trait[0]).__name__
+                        self.text("* ").\
+                            text(category).\
+                            text(": any of: ").\
+                            line(", ".join([t.name for t in trait]))
+
+                for trait in habitat.biomeFilter.prohibited:
+                    category = type(trait[0]).__name__
+                    self.text("* ").text(category).text(": not: ").line(trait.name)
+
+                self.outdent()
+
+                self.text("* seasons: ").line(", ".join([s.name for s in habitat.seasons]))
+                self.text("* group: ").line(habitat.group.name)
+                self.text("* location: ").line(habitat.location.name)
+                self.text("* scarcity: ").line(habitat.scarcity.name)
+                self.outdent()
+
+            self.outdent()
+            self.line()
diff --git a/tungston/modpack/mysteriousisland/biomecatalog_test.py b/tungston/modpack/mysteriousisland/biomecatalog_test.py
new file mode 100644
index 0000000..f13729a
--- /dev/null
+++ b/tungston/modpack/mysteriousisland/biomecatalog_test.py
@@ -0,0 +1,7 @@
+import tungston.modpack.mysteriousisland.biomecatalog
+
+
+# Class ############################################################################################
+
+def test_syntax():
+    pass
diff --git a/tungston/modpack/mysteriousisland/depositcatalog_test.py b/tungston/modpack/mysteriousisland/depositcatalog_test.py
new file mode 100644
index 0000000..74f962d
--- /dev/null
+++ b/tungston/modpack/mysteriousisland/depositcatalog_test.py
@@ -0,0 +1,7 @@
+import tungston.modpack.mysteriousisland.depositcatalog
+
+
+# Class ############################################################################################
+
+def test_syntax():
+    pass
diff --git a/tungston/modpack/mysteriousisland/mineralcatalog_test.py b/tungston/modpack/mysteriousisland/mineralcatalog_test.py
new file mode 100644
index 0000000..ce62f21
--- /dev/null
+++ b/tungston/modpack/mysteriousisland/mineralcatalog_test.py
@@ -0,0 +1,7 @@
+import tungston.modpack.mysteriousisland.mineralcatalog
+
+
+# Class ############################################################################################
+
+def test_syntax():
+    pass
diff --git a/tungston/modpack/mysteriousisland/mobcatalog.py b/tungston/modpack/mysteriousisland/mobcatalog.py
index e462dcf..4fb78a0 100644
--- a/tungston/modpack/mysteriousisland/mobcatalog.py
+++ b/tungston/modpack/mysteriousisland/mobcatalog.py
@@ -23,22 +23,22 @@ import tungston.core.season           as SE
 CATALOG = MobCatalog([
     MobPlacement(Mob("armadillo", "minecraft:armadillo", AC.NOCTURNAL),
         Habitat(
-            altitudes   = AL.LOWLANDS,
+            altitude    = AL.LOWLANDS,
             biomeFilter = BF([HE.SUBTROPICAL, HU.DRY]),
             seasons     = SE.SUMMER,
             group       = GR.merge(GR.SOLO, GR.PAIR),
             scarcity    = SC.COMMON
         ).derive(
-            altitudes = AL.UPLANDS, scarcity = SC.SPARSE,
+            altitude = AL.UPLANDS, scarcity = SC.SPARSE,
         ).derive(
-            altitudes = AL.LOWLANDS, seasons = SE.excluding(SE.SUMMER), scarcity = SC.UNCOMMON,
+            altitude = AL.LOWLANDS, seasons = SE.excluding(SE.SUMMER), scarcity = SC.UNCOMMON,
         ).derive(
-            altitudes = AL.UPLANDS, scarcity = SC.UNUSUAL,
+            altitude = AL.UPLANDS, scarcity = SC.UNUSUAL,
         )
     ),
     MobPlacement(Mob("black_bear", "bearminimum:black_bear"),
         Habitat(
-            altitudes = AL.span(AL.LOWLANDS, AL.ALPINE),
+            altitude = AL.span(AL.LOWLANDS, AL.ALPINE),
             biomeFilter = BF([HE.TEMPERATE, FL.FOREST]),
             seasons = SE.AUTUMN,
             group = GR.merge(GR.SOLO, GR.FAMILY),
@@ -59,7 +59,7 @@ CATALOG = MobCatalog([
     ),
     MobPlacement(Mob("blaze", "minecraft:blaze"),
         Habitat(
-            altitudes = AL.PLUTONIC,
+            altitude = AL.PLUTONIC,
             location  = LO.CAVE,
             group     = GR.SOLO,
             scarcity  = SC.UNCOMMON
@@ -68,7 +68,7 @@ CATALOG = MobCatalog([
     MobPlacement(Mob("bogged", "minecraft:bogged")),
     MobPlacement(Mob("brown_bear", "bearminimum:brown_bear"),
         Habitat(
-            altitudes   = AL.span(AL.LOWLANDS, AL.ALPINE),
+            altitude   = AL.span(AL.LOWLANDS, AL.ALPINE),
             biomeFilter = BF([HE.BOREAL, (FL.CANOPY, FL.FOREST)]),
             seasons     = SE.AUTUMN,
             group       = GR.SOLO,
@@ -82,19 +82,19 @@ CATALOG = MobCatalog([
     MobPlacement(Mob("bulwark", "immersiveengineering:bulwark")),
     MobPlacement(Mob("camel", "minecraft:camel", AC.CREPUSCULAR),
         Habitat(
-            altitudes = AL.span(AL.DUNES, AL.LOWLANDS),
+            altitude = AL.span(AL.DUNES, AL.LOWLANDS),
             biomeFilter = BF([HE.TROPICAL, HU.DRY, FL.BARREN, SO.SANDY]),
-            seasons = SE.WET,
+            seasons = SE.DRY,
             group = GR.FAMILY,
             scarcity = SC.UNUSUAL,
         ).derive(
-            seasons = SE.DRY, scarcity = SC.RARE
+            seasons = SE.WET, scarcity = SC.RARE
         )
     ),
     MobPlacement(Mob("cat", "minecraft:cat")),
     MobPlacement(Mob("cave_spider", "minecraft:cave_spider"),
         Habitat(
-            altitudes = AL.span(AL.OVERBURDEN, AL.HILLS),
+            altitude = AL.span(AL.OVERBURDEN, AL.HILLS),
             location  = LO.CAVE,
             group     = GR.SOLO,
             scarcity  = SC.UNCOMMON
@@ -102,7 +102,7 @@ CATALOG = MobCatalog([
     ),
     MobPlacement(Mob("chameleon", "cold_sweat:chameleon"),
         Habitat(
-            altitudes = AL.span(AL.DUNES, AL.UPLANDS),
+            altitude = AL.span(AL.DUNES, AL.UPLANDS),
             biomeFilter = BF([HE.TROPICAL, HU.WET, [FL.CANOPY, FL.FOREST]]),
             seasons = SE.WET,
             group = GR.merge(GR.SOLO, GR.FAMILY),
@@ -119,7 +119,7 @@ CATALOG = MobCatalog([
     ),
     MobPlacement(Mob("chicken", "minecraft:chicken"),
         Habitat(
-            altitudes = AL.span(AL.LOWLANDS, AL.UPLANDS),
+            altitude = AL.span(AL.LOWLANDS, AL.UPLANDS),
             biomeFilter = BF([HE.TROPICAL, HU.WET, FL.within(FL.CANOPY, FL.CLEARING)]),
             seasons = SE.SUMMER,
             group = GR.TROUP,
diff --git a/tungston/modpack/mysteriousisland/mobcatalog_test.py b/tungston/modpack/mysteriousisland/mobcatalog_test.py
new file mode 100644
index 0000000..9c71fa0
--- /dev/null
+++ b/tungston/modpack/mysteriousisland/mobcatalog_test.py
@@ -0,0 +1,7 @@
+import tungston.modpack.mysteriousisland.mobcatalog
+
+
+# Class ############################################################################################
+
+def test_syntax():
+    pass
diff --git a/tungston/modpack/mysteriousisland/runner.py b/tungston/modpack/mysteriousisland/runner.py
index cf2cfdb..513ecf3 100644
--- a/tungston/modpack/mysteriousisland/runner.py
+++ b/tungston/modpack/mysteriousisland/runner.py
@@ -7,6 +7,7 @@ from tungston.modpack.mysteriousisland.mobcatalog import CATALOG as mobs
 from tungston.generator.markdown.reportwriter import ReportWriter
 from tungston.generator.markdown.biomereport import BiomeReport
 from tungston.generator.markdown.mineralreport import MineralReport
+from tungston.generator.markdown.mobreport import MobReport
 
 import sys
 
@@ -23,7 +24,8 @@ class Runner(BaseRunner):
     def _command_writeReports(self):
         writer = ReportWriter([
             BiomeReport(self.world),
-            MineralReport(self.world)
+            MineralReport(self.world),
+            MobReport(self.world),
         ])
         writer.write(REPORT_PATH)