36 



MEMOIRS OF THE UNIVERSITY OP CALIFORNIA. 



35 



Neural 



(After 



Fig. 35. Ichthyosaurus quadrisdssus Quensteclt. 



arch with united zygapophyses, X %. 



Fraas.) 

 Fig. 36. Cymbospondylus petrinm Leitly. Neural arch 



of 5th cervical vertebra, anterior view, X %. 



noted by Fraas (1891, p. 39), the facets of the zygapophyses in 

 atavus differ from those of Ichthyosaurus in being distinctly separate;! 

 and not situated in the same plane (figs. 34a to 34c). In the Jurassic forms 

 the two faces are brought together at the median line and unite to form a single 



face (fig. 35). In all of American 

 Triassic forms (figs. 31-33 and 36) 

 the zygapophysial facets are large, 

 distinctly separated, and inclined 

 away from each other in the cervical 

 and anterior dorsal region. In the 

 posterior dorsal and caudal regions 

 the facets may he considerably re- 

 duced and fall into nearly the same 

 plane. This difference is evidently 

 due to the fact that in the development 

 of a broad, vertical sculling organ, 

 the principal movements of the body 

 have been sideways, and a union of the zygapophysial facets in one plane fa- 

 cilitated this movement. The normal form of zygapophysial articulation in 

 which the anterior facets look toward each other would interfere to some extent 

 with this motion. In the evolution of the group the tendency to change the 

 zygapophyses naturally appeared first in the caudal region and was gradually 



carried forward toward the neck region. In the Mid- 

 dle Triassic forms the sculling tail was well devel- 

 oped and the zygapophyses of the posterior region of 

 the column had already been considerably modified. 

 In the cervical, and to some extent in the anterior 

 dorsal region, the modification had not been very 

 strongly expressed. The evidence of the zygapophy- 

 ses can scarcely be interpreted as pointing toward 

 any other than a land or shore type as the ancestral 

 form of the group. 



H-jipocentral Elements. The inferior or hypocen- 

 tral elements of the caudal region furnish a most 

 characteristic feature of all Triassic genera, and rep- 

 resent a less advanced stage in the development of 

 the ichthyosaurian tail fin than in the post-Triassic 

 forms. In every case where they have been observed the caudal hypocentra 

 are united ventrally to form a long median stem (fig. 37), giving the inferior 

 arches the Y-shaped form seen in the lower arches of the caudal series in most 

 reptiles. In the type specimen of Delphinosaurus perrini (pi. 7, fig. 2) the 



37 



Fig. 37. Delphinosaurus perrini 

 Merriam. Inferior arch of an- 

 terior caudal vertebra, X %. 



