Fil 2 SS ROD ILE Si 4 Olea SCD EAINGIES) 
The degeneration of the limbs and the phalanges is exactly the 
same as took place in the mammals that have become aquatic 
in their habits ; the seals, walruses and the whales. 
The Jcthyosauria are known only from the marine rocks of 
the Mesozoic time and chiefly from the Liassic division. Speci- 
mens are known from North America, Europe and England ; 
Queensland, Australia; New Zealand and the East Indies. 
Mixosaurus, from the Triassic of Besamo in Italy, is one of the 
oldest as well as one of the most important of the group. The 
animal was small, about three feet long, the head was propor- 
tionately long and there were few teeth; the legs were long and 
well developed, typically those of a land-living form. 
Icthyosaurus —TVhis genus contained a very large number 
of species; it is generally divided into two groups, the Longr- 
pinnatti and the Latipinnatt, accordingly as the limbs, which 
in the whole genus have been reduced to the condition of 
paddles, are long and slender with few, five or less digits, 
or broad and short with more than five digits. Typical of 
the first division are: /. quadricissus, I. tricossus, I. tenuirostyts, 
I. longifrons, etc.; of the second group, /. communis, I. trigonus, 
L. leptospondylus, etc. ) 
The genus is found most commonly in the rocks of the Lias. 
From the Wellendolomite of the Black Forest and the Muschel- 
kalk of Crailsheim of Wurtemberg comes one of the oldest as 
well as one of the largest of the genus, /. afavus, about thirty 
feet long. From the Trias of Spitzbergen have been collected 
two species, /. polaris and I. Nordenskioldi; the first was very 
large, while the second was much smaller... From the Lias of 
England come the best preserved specimens that we have. 
Especially is this true of the Lyme-Regis region of Dorsetshire. 
l. communis from this region belongs to the first division, the 
Latipinnati,; it was one of the largest of the genus. To the same 
division belong from this region: JL. zntermedius and I. breviceps ; 
to the second class, among others: /. platyodon and I. tenuzrost- 
vis. The first was a giant even among these large forms, but the 
second was quite small and delicate in structure. The paddles 
