.MKKI.MA.M: TKIASSIC K'HTII YOSAURIA. 29 



case, the dentition of Cuiboxj><i<l/jlns is not differentiated more than that of 

 Ichthyosaurus, it may be that the differentiation of the rnixosaur dentition was 

 acquired in adaptation to a peculiar food habit; while the thecodont insertion, 

 here as in most of the Triassic forms, maybe a persistent or primitive character. 



VERTEBRAE. 



Vertebral Formula. In the typical Ichthyosauria of the Jurassic the num- 

 ber of vertebrae in the spinal column shows considerable variation. In the 

 genus Ichthyosaurus variation occurs principally in the posterior caudal region 

 through the addition of small centra in the pinnigerous portion of the tail. The 

 presacrals in Ichthyosaurus pretty constantly number between 40 and 46 with 

 some slight variation from this number. An exception is found in the pecu- 

 liarly specialized /. platydacti/lus Broili of the Cretaceous, in which the pelvis 

 appears to have been near the 54th centrum. In Baptanodon reedi Gilmore 

 the region of the pelvis is indicated at the 49th or 50th centrum, as the trans- 

 verse processes reach their lowest position on the sides of the centra at this 

 point. In Optlialmosaurus there are 40 presacrals, according to Andrews. 13 



Of the Triassic genera the complete presacral segment of the column is 

 known only in Mixosaurus and Cymbospondylua, Judging f rom Repossi's fig- 

 ures there are between forty-five and fifty presacrals present in Mixosaurus. 

 In a specimen of Cymbospondylus (pi. 7, fig. 1; no. 9950), including the com- 

 plete presacral region, there are not less than 65 presacrals. In the type of 

 Dclphinosaurus (pi. 7, fig. 2) there are 40 presacrals present, and enough of 

 the cervical region is absent to increase the number to between 45 and 50. In 

 specimen 10998 (pi. 7, fig. 3) the vertebral column is complete to the posterior 

 part of the skull, and there are about 40 presacrals present. As the region of 

 the back of the skull is imperfectly preserved, there are probably several verte- 

 brate not recognized, so that the total may be supposed to be the same as in the 

 type specimen. In Shastasaurus osmouti (fig. 57) there are at least thirty- 

 six vertebrae anterior to the region in which the rib articulation moves down 

 from the upper portion of the lateral face of the centrum. As this downward 

 movement occurs in all other ichthyosaurians near the middle of the dorsal 

 region and generally fifteen or more vertebrae anterior to the pelvis (Delphin- 

 osaurus 15, Cymbospondylus 19 or more. Ichthyosaurus 15 or more) at least fif- 

 teen centra may be added to the thirty-six known in Shastasaurus making the 

 number near 50. It appears then that in the known Triassic forms the number 

 of presacrals is generally somewhat larger than in the typical Ichthyosaurus. 

 That the number should be larger in these forms than in most of the later ich- 

 thyosaurs is rather remarkable, since the primitive reptilian types as the Diap- 



is Andrews, C. W., Geol. Mag., Dec. 5, vol. 4, p. 205. 



