30 MEMOIRS OF THE UNIVERSITY OF CALIFORNIA. 



tosauria and the Phytosauria, which have been suggested as ancestral to the 

 Ichthyosauria, have in nearly all cases less than thirty presacrals. 



The total number of vertebrae in the caudal region including the smaller 

 centra at the end of the pinnigerous portion seems to be very closely repre- 

 sented in a DelpMnosaurus specimen (no. 10998), and is approximately 75. 

 In one of the Milan specimens of Mixosaurus the writer has counted about 

 seventy-six vertebrae behind the posterior limb. In a large specimen of C'/jiit- 

 bospondylus (fig. 132, no. 9947) a continuous series of 54 vertebrae is present 

 behind the pelvis, and a part of the distal region of the tail is absent. 



' Pigs. 17-20. Shastasaurus osmonti Merriam. Outlines of cervical and dorsal vertebrae, X 1 /-!- Fig. 

 17, anteroposterior section of 36th centrum; fig. 18, anteroposterior profile of 4th centrum; fig. 

 19, anteroposterior profile of 12th centrum; fig. 20, anteroposterior profile of 36th centrum. 



In the caudal region the number of vertebrae anterior to the characteristic 

 bend in the tail is, so far as known, nearly the same as in the later forms, or in 

 some cases somewhat smaller. In the Milan Mixosaurus specimen possessing 

 seventy-six caudals. the bend occurs near the twenty-fifth centrum behind the 

 pelvis. On one specimen of DelpMnosaurus (pi. 7, fig. 2) it is situated be- 

 tween the 25th and 30th vertebrae behind the pelvis, and in another (pi. 7, fig. 

 3) it appears to be between the 25th and 35th centra. In Cymbospondylus 

 (fig. 132) the bend in the caudal region occurs at the 30th to 35th centrum behind 

 the pelvis as it is situated here. This number is nearly the same as the number 

 of centra that ordinarily intervenes between the pelvis and the downward bend 

 of the tail in Ichthyosaurus. 



Form of Centra. The individual vertebral centra of all Triassic ichthyo- 

 saurians are biconcave as in later types. The biconcavity is so deep in some 

 forms, as in the posterior dorsals of Cynibospondylus (fig. 24) or in the 

 caudals of some of the small species from the Upper Triassic of Shasta 

 County, that the middle portion of the centrum is perforated by a very small 

 canal. The same character has been noted in the vertebrae of Mixosaurus ( ?) 

 atavus by Fraas (1891, pp. 38 and 39) and was described by von Meyer 14 for 



i* von Meyer, H., Palaeontographica, Bd. 1, p. 253. 



