MERRIAM: TRIASSIC ICHTHYOSAUBIA. 113 



posterior limit of the cervical region can lie made, eleven vertebrae is a rela- 

 tively large number for this portion of the column. 



The atlas and axis are clearly shown in position in several specimens. (See 

 pi. 8, also text-figure, 58.) The centrum of the atlas is as wide, and in some 

 cases almost as high, as that of the axis. Its auleroposterior diameter is con- 

 siderably less than that of the axis. The anterior face is strongly convex, the 

 convexity reducing the lateral and inferior borders to one-half or one-third the 

 anteroposterior diameter of the centrum. The anterior face shows no depres- 

 sion into which the basioccipital might be received. The anterior face of the 

 atlas is received in a deeply excavated articular face on the basi occipital. The 

 posterior face of the atlantar centrum and the anterior face of the axis are 

 both concave, and there is no suggestion of fusion of the t\vo centra such as 

 occurs in the later ichtliyosaurs. The elements of the upper arch of the atlas 

 as far as known are not materially different from those on the atlas of Ichthy- 

 osaurus. 



The adjacent inferior borders of the atlas and axis may be obliquely trun- 

 cated toward the plane of contact, for the intercalation of an intercentrum. 

 There seems also to have been a small intercentrum between the third centrum 

 and the axis, but none was present behind the third. 



On the axis of no. 9950 there is a short diapophysis but no distinct parapo- 

 physis. In no. 9943 (fig. 58, p. 48) both diapophysis and parapophysis are pres- 

 ent. On centra three and four of no. 9154 parapophyses of considerable size are 

 present and small, nearly round articular facets are developed for the lower 

 rib heads. On centrum six the parapophysis is very rudimentary, and on what 

 appears to be the seventh it is absent. In no. 9950 the diapophysis on the 

 fourth centrum is 43 mm. high, the parapophysis 17 mm. On the eighth cen- 

 trum the corresponding dimensions are 45 mm. and 8 mm., and the parapo- 

 physis is generally much reduced. On the eleventh centrum there may be a 

 minute rudiment of a parapophysis on the anterior margin. In specimen 9943 

 relatively large parapophyses are present back to the twelfth centrum (fig. 58, 

 p. 48). On the thirteenth there is a trace of a parapophysis on one side only. 

 In this specimen the parapophyses increase somewhat in size toward the dor- 

 sal region, and are larger both absolutely and relatively on the twelfth com- 

 pared with the size of the diapophyses than they are on the fifth and sixth centra. 

 The form and dimensions of the diapophyses remain about the same, back to 

 the twelfth centrum. On the thirteenth they increase markedly in height, but 

 the increase is not sufficient to make the height of the diapophysis on the thir- 

 teenth centrum equal the complete height of the diapophysis and parapophy- 

 sis on the centra immediately preceding. The anterior cervical rib heads are 

 divided proximally by a deep notch forming two distinct heads. 



In those cervicals in which the ends of the centra are visible the degree of 



