94 MEMOIRS OP THE UNIVERSITY OP CALIFORNIA. 



<it(i en s type are deeply biconcave and do not show a marked flattening of the 

 anterior and posterior faces near the periphery (figs. 117r/ and 117^). An ex- 

 ception may appeal- in some of the distal caudals. 



Of the vertebral arches the only one known to the writer is the upper arch 

 (figs. 34a-34c) described by Fraas (1891, Taf. 3, figs. 6a-r). This is an ante- 

 rior dorsal arch of much the same type as in C ynibospondylus. The zygapo- 

 physial facets are very primitive in that they are still distinctly separated, they 

 do not fall into the same plane, and they are still far from coming into a hori- 

 zontal position. 



Limb and Arch Elements. The only limb bones of Jl/.(?) atai'iix known, 

 include a humerus described by Fraas (1891, Taf. 3, fig. 5), and several' sep- 

 arated podial elements figured by Quenstedt (1852, Taf. 6, figs. 9-10). The 

 humerus is a slender but not greatly elongated element with a very high pec- 

 toral ridge. Though somewhat damaged distally it does not appear to be more 

 slender than the propodial element in a number of other ichthyosaurians (see 

 von Huene 1902, p. 11 ). The other known limb elements are possibly mesopod- 

 ials. 



A number of bones from the arches are known but have not yet been de- 

 scribed. 



Jdtcs and Dentition. Of the skull only a few fragments consisting of jaws 

 are described. Excepting the somewhat greater prominence of the surface 

 sculpture, and the insertion of the teeth these elements are much as in 

 Ichthyosaurus. 



The dentition which is fortunately known in several jaw fragments presents 

 interesting characters which suggest that Mixosaurus is possibly a closely re- 

 lated type. In the several specimens which Fraas had before him there were 

 present a portion of an anterior end of a jaw with slender conical teeth (fig. 

 15, p. 28) and a part of a jaw in which there are two teeth with low blunt 

 crowns (fig. 16, p. 28). In one of the two teeth in the second fragment the 

 crown is considerably compressed laterally, and is a little lower than in the 

 other. In both teeth the crown shows surface markings on the enamel, and is 

 apparently not reduced by wear. The bases exhibit strong folds of the dentine, 

 and appear to be partly 'in pits. These specimens apparently indicate a dif- 

 ferentiated dentition similar to that seen in Mixosaurus. The structure and 

 insertion of these teeth are somewhat like that of the ichthyosaurians described 

 from the Trias of Spitsbergen (see p. 149). 



MIXOSAURUS Baur. 



Ichthyosaurus (Bassani), Atti della Soc. Ital. di Sc. Nat., vol. 29, p. 15. 

 Mixosaunix Baur. Ber. d. 20 Versam. d. Oberh. (ieol. Ver., 1887, p. 19. 



Distal end of caudal region slightly bent but not sharply decurved as in 

 Ichthyosaurus, broadened by elongation and erection of the vertebral arches. 



