MERRIAM: THIASSIC ICHTHTOSAUBIA. l.'i!) 



the neural arches. In the anterior region of the column the diapophyses are 

 relatively short, and well developed parapophyses are present below them. In 

 proceeding backward in the series the diapophyses gradually elongate, and in 

 the middle dorsal region develop a knee-like bend at about the middle of their 

 height, the lower portion of the apophysis swinging forward almost to the 

 anterior margin of the centrum. Behind the anterior ten or eleven vertebrae 

 the parapophyses are reduced and were not functional. The last minute trace 

 appears on one side of the eighteenth centrum. 



The upper arches of the vertebrae in the most anterior cervical region pos- 

 sess slender spines with a round cross-section. The spines of the dorsal arches 

 are thick compared with those of Ichthyosaurus, but they are more compressed 

 laterally than in S. altispinus or S. careyi. 



The ribs are rather heavy and tend to develop the outward curvature of the 

 upper portion of the articular face, permitting a rocking movement, but much 

 less strongly than in S. altispinus (see figs. 66 and 66&, p. 52) or S. careyi. 



In specimen 9081 there is present the posterior portion of a cranium of 

 S. oion/i with the cervical and anterior dorsal vertebrae, the pectoral girdle, 

 and portions of both anterior limbs. The limb and arch elements are slightly 

 smaller than those of the type specimen, but the general form and proportions 

 are the same. In this specimen the series of cervical vertebrae seems to be 

 complete, though the anterior individuals are displaced somewhat. Immedi- 

 ately behind the articular face of the basioccipital is a small, thin centrum 

 representing the atlas. One surface shows a funnel-like concavity which ex- 

 tends inward almost from the margin. The opposite side is gently convex, ex- 

 cepting a small, shallow, central concavity covering not more than one-third of 

 the area. The widely concave side apparently represents the posterior face, 

 the anterior face of the centrum considered as the axis being also deeply con- 

 cave. Its funnel-shaped form is, moreover, not the form of excavation that 

 would probably develop in articulation with the basioccipital. The posterior 

 articular face of the basioccipital is strongty convex, and the middle of the 

 face evidently rested in the small concave middle area of the anterior side of 

 the atlas. 



The atlantar centrum differs from that of Cymbospondylus in the develop- 

 ment of the slight concavity on the otherwise convex anterior face, and in the 

 apparent absence of distinctly marked lateral faces. The addition of a cer- 

 tainly known atlas to the vertebral series of Shastasaurus suggests that, as 

 indicated in the original description, the number of cervicals with attached 

 intercentra may be larger than in Ichthyosaurus. 



A valuable specimen (no. 9608; pi. 17, fig. 1) found during the season of 

 1903 appears to correspond to S. osmonti, and shows the previously unknown 

 vertebrae and ribs of the posterior dorsal, sacral, and anterior caudal regions, 



