26 



J. C. Merriam — Triassie Ichthyosauria. 



podial and phalangeal regions, as far as known, are rounded and 

 have deeply excavated borders, showing that they lay in heavy 

 pads of cartilage. Judging from the character of the carpus 

 in Delphinosaurus and in Merriamia, the large radiale in 

 S. osmonti indicates that the anterior digit of the manus was 

 relatively larger and the third smaller in Shastasaurus than in 

 the other forms. The limb would in that case be reduced almost 

 to a two-lingered type. 



Degree of differentiation. 



The degree of differentiation shown in the three or four 

 types of limb structure known in the Triassie Ichthyosaurs 



o O 



Fig. 5. — Ichthyosaurus quadricissus. Anterior limb. x f . After Fraas. 

 Fig. 6. — Ichthyosaurus conybeari. Anterior limb. After Lydekker. 

 Fig. 7. — Baptanodon marshi. Anterior limb. After Knight. 



appears quite remarkable when we compare it with what we 

 find in the Jurassic genera. Three types of limb structure are 

 known from the Jurassic. The most specialized of these is 

 found in Baptanodon, (fig. 7) and Opthalmosaurus with three 

 very short elements in the epipodial region, five or more digits, 

 and discoidal phalanges. In Ichthyosaurus, possessing two 



