84 MEMOIRS OF TI1K TXIVKKSITY OF CALIFORNIA. 



shown more recently that the type of limb structure in Shastasaurus is also 

 quite different from that in the other divisions of the Ichthyosauria. 



Of the various groups of characters in which the Ichthyosauria show dif- 

 ferences, the strongest contrasts are found in the (1) limbs, (2) pelvic arch, 

 (3) caudal fin, (4) caudal chevrons, (5) articulation of dorsal ribs, and (6) the 

 dentition. The variation in most of these characters may apparently be attrib- 

 uted to adaptation of the peculiar type expressed in the Ichthyosauria. 



If the ichthyosaurs be taken as a whole, one of the most striking contrasts 

 is shown in a comparison of the Triassic and post-Triassic forms, particularly 

 in the characters just mentioned, as is indicated by the following table : 



JURASSIC 

 TRIASSIC CRETACEOUS 



v 



I I fill 111 



Ilislistf 



Rib articulation mainly single x x x x x 



Rib articulation mainly double x x 



Distal end of caudal series strongly decurved ? ? ? x 



Caudal vertebrae with reshaped lower arches x x ? x x x 



Pelvis plate-like, not reduced x x ? x x x 



Pelvis much reduced x x x 



Scapula much broadened distally x x x x x ? 



Epipodials separate and relatively long x x x x x* x 



Hyperphalangy developed x x 



Phalangeal elements sometimes shafted '? x x ? x 



Phalanges polygonal or discoidal ? f x x x x 



Phalanges reaching double the normal formula 



for diapsidan reptiles ? ? ? x ? 



Teeth numerous x x ? ? x 



Teeth differentiated x ? ? ! 



Teeth in pits x* x ? ? f 

 * In part. 



