152 MEMOIRS OF THE UNIVERSITY OF CALIFORNIA. 



pears to represent the radius was evidently not deeply concave anteriorly, and 

 the ulna was possibly, not constricted medially. 



The elements of the mesopodial region are angular instead of rounded. 

 There are in the proximal row three elements closely resembling the radiale, 

 intermedium and ulnare of Mixosaurus. Behind the ulnare there is a portion 

 of a third element which may represent the pisiform of Mixosaurus. Articu- 

 lating with the ulnare there are two elements in the distal row of the carpus. 

 The anterior of these two shows a small contact with the intermedium. The 

 distal end of the intermedium is not complete. 



With this species there may also be placed tentatively several series of caudal 

 vertebrae representing a primitive type of caudal fin. One of these specimens 

 (no. 10624; see pi. 10, fig. 1) is a part of an individual somewhat larger than 

 that represented by the humerus standing as the type of this species. Other 

 specimens showing much the same type of caudal vertebrae as those in no. 

 10624 belong to smaller individuals. 



The vertebral centra in the caudal series mentioned above are high and nar- 

 row. They vary somewhat in anteroposterior diameter in different specimens. 

 On specimen 10624 well marked diapophyses are present. The anterior and 

 posterior articular faces are not well shown, but as nearly as can be determined 

 they are not deeply concave from the margins inward, but are probably of the 

 form seen in the caudals of Cymbospondylus petrinus (pi. 9, fig. 8&). 



The upper arches are greatly elongated, the height amounting to more than 

 two and one-half times that of the centrum in one case. Though much elon- 

 gated the neural arches are not greatly compressed laterally. Zygapophysial 

 notches are noticeable particularly on the posterior margins of these arches. 



The lower arches are also very long, their length almost equaling that of the 

 neiiral arches. Their attachment to the centra cannot be clearly made out in 

 e\ T ery case. In one instance a suture between an articular head of a lower arch 

 and the centrum can be clearly seen. In other cases the lower arches may pos- 

 sibly be fused with the centra, as in the mosasaurian genus Clidastes. 



ICHTHYOSAURUS Koenig. 



Single articulation of presacral ribs found only in the posterior dorsal re- 

 gion. Hypocentral elements of caudal region not meeting to form Y-shaped 

 arches. Distal region of caudal series sharply decurved. Lateral zygapophy- 

 sial faces united to form single anterior and posterior facets. Scapula narrow 

 distally. Pelvic elements much reduced. Epipodial elements short, rarely sep- 

 arated by a cleft. Phalangeal elements short, with angulated or rounded mar- 

 gins. Maxillary relatively short compared with the premaxillary. Orbits very 

 large. Postorbital bar narrow. Teeth numerous, isodont, situated in open 

 grooves. 



