38 



MEMOIRS OF THE UNIVERSITY OF CALIFORNIA. 



strongly convex, the inferior and lateral borders being much thinner than the 

 superior side. The anterior face of the atlas shows no concave face into which 

 the basioccipital might be received, but the posterior face of the basioccipital 

 is deeply excavated for the reception of the atlas. The narrowed lateral bor- 

 ders have elevated anterior and posterior edges and show no indication of the 

 presence of rib articulations. The upper arches of the atlas are formed much 

 as in Ichthyosaurus. 



In Shastasaurus (fig. 57) the form of the atlas is much as in Cymbospon- 

 dylus, though the lateral borders are evidently not as sharply set off from the 

 anterior face by an elevated anterior edge. A portion of an upper arch, appar- 



39 



40 



Fig. 38. Cymbospondylus petrinus Leidy. Atlas and axis, lateral view, X %. 1, atlas; 2, axis. 

 Fig. 39. Ichthyosaurus longifrons Owen. Atlas and axis, lateral view, X %. 1, atlas; 2, axis. 



(Adapted from Owen.) 



Fg. 40. Ichthyosaurus platydactylus Broili. Atlas, axis, and 3rd cervical, lateral view, X % 1 

 and 2, fused atlas and axis; 3, 3rd cervical centrum. (After Broili.) 



ently belonging to the atlas in the type specimen of S. alcxandrae, resembles the 

 neurocentral elements of Ichthyosaurus. 



The axis of Cymbospondylus is in general like the succeeding cervicals, 

 excepting in its marked inferior truncation for the hypocentral elements. It 

 is characterized in some of the specimens examined (pi. 8, fig. 6) by the absence 

 of parapophyses. The diapophyses of the axis are like those of the third cen- 

 trum. The third vertebra shows large parapophyses. In Shastasain-iis the 

 rib articulation of the axis resembles that of the corresponding element in the 

 later ichthyosaurs, and large parapophyses as well as diapophyses are present. 

 Excepting in the two genera described above, the atlas and axis are not well. 

 known in the Triassic forms. 



In the genus Ichthyosaurus the first two elements of the vertebral column 

 are characterized generally by partial or complete fusion with each other and, 

 so far as the information is available, there seems to be no indication of an 

 intervertebral space remaining. The atlas and axis are also of nearly the same 

 size (fig. 39). 



In the latest members of the ichthyosaur group, as in Baptanodon, the 

 fusion of the first two elements is complete, and the hypocentral element be- 



