96 MEMOIRS OP THE UNIVERSITY OF CALIFORNIA. 



with Ichthyosaurus of the Jurassic, Mixosaurus shows decidedly less of the 

 extreme specialization in adaptation to aquatic life. Although not only a 

 typical aquatic form, but already far advanced along this line of specialization, 

 in nearly all respects in which comparison can be made, it is decidedly more 

 like a shore animal than are the Jurassic ichthyosaurs. This incompleteness of 

 adaptation is seen particularly in the organs of locomotion, but is also expressed 

 more or less distinctly in other portions of the body. Though Mixosituni* is 

 probably not more primitive than the Middle Triassic Ichthyosauria of America, 

 it is evidently one of the most generalized members of the group. 



Some characters in which Mixosaurus departs from the typical ichthyosaurs 

 possibly represent specializations of this particular genus rather than per- 

 sistent primitive characters. The uniformly small size, in itself a primitive 

 character, has however tended to keep the North-Italian forms more generalized 

 in certain respects than some of the larger forms of the American Trias. 



121 122 123 124 125 126 



Figs. 121 to 126. Mixosaurus cornalianus (Bassani). Pigs. 121 to 125, after Bepossi. Fig. 126, 

 from rough sketch by the author. Figs. 121 to 125, natural size. Fig. 126, about natural size. 



Figs. 121 and 122, dorsal vertebrae; fig. 123, neural arch of a dorsal vertebra; fig. 124, caudal 

 centrum; figs. 125 and 126, lower arches of caudal vertebrae. 



Vertebral Column. Several of the best specimens taken together represent 

 practically the whole of the spinal column, though in most specimens it is 

 hardly possible to obtain thoroughly satisfactory information concerning all 

 regions of the series. According to Repossi, the vertebral column consists of 

 about one hundred to one hundred and five vertebrae. Judging from his 

 figure (1902, tav. 8, fig. 1) of a nearly complete specimen there were between 

 forty-five and fifty presacrals. In one of the Milan specimens the writer has 

 counted about seventy-six vertebrae behind the posterior limb. These two 

 estimates would seem to bring the total number up to one hundred and twenty. 

 The number of presacrals appears approximately the same as that in Ichtlit/o- 

 Mtiirus, though a complete specimen might show a few more than the average 

 for the later genus. This is in strong contrast to the presacral formula of 

 Cymbospondylus, which comprises more than sixty-five vertebrae. 



As in all other ichthyosaurians the centra are biconcave. The concavity 

 appears to extend well out toward the margins (fig. 121), and is not expressed 

 mainly near the middle of the centrum,excepting possibly in the caudals. 



