50 



MEMOIRS OF THE UNIVERSITY OF CALIFORNIA. 



and situation occur in the middle caudal region. The centrum shows a cen- 

 tral perforation. Such perforations appear to be more common in the cau- 

 dal centra than in other regions of the column, but the nearly circular cross- 

 section is not characteristic of the caudal region. If the centrum is an ante- 

 rior dorsal the apophysis present is possibly the parapophysis, the tubercle 

 having rested on the base of the upper arch. 



That forms of rib articulation as different as those discussed above for 

 M. ( "?) atavus occurred in the same region of the vertebral column in variations 

 of the same species or even of the same genus seems improbable. It would 

 therefore appear that we are dealing either with different parts of the verte- 

 bral column of similar individuals, or with quite different generic or specific 



62 



63 



64a 



64b 



Figs. 61-646. M ixosau rus ( ? ) atavus (Quenstedt). Vertebrae natural size. Fig. 61, anterior dorsal or posterior 

 cervical; fig. 62, middle caudal or middle dorsal; fig. 63, posterior dorsal; figs. 64a and 64ft, from posterior 

 dorsal or most anterior caudal region; figs. 61, 64o, and 646, from rough sketches by the author; figs. 62 

 and 63, after Fraas; figs. 61-63, from specimens in the Ko'nig. Naturalien-Cabinet, Stuttgart; figs. 64a and 

 646, from a specimen in the Polytechnikum, Zurich. 



types occurring together, as do Toretocnemus and Merriamta in the Upper 

 Triassic of California. Apparently the only other possible situation in which 

 either of the types of vertebrae referred to the posterior dorsal region could 

 be placed would be the anterior caudal region; and that either form could be 

 referred to this series is improbable as chevron facets are not present, and all 

 other Triassic forms as well as most Jurassic ichthyosaurs possess well devel- 

 oped caudal hypocentra. 



In the case of Mixosaurus, in which a division of the rib heads is supposed 

 to occur in the caudal region, Yakowlew (1902, 2) has suggested that the rib 

 articulations were originally single-headed, and that a division into two heads 

 began in the caudal region owing to the influence of the peculiarly powerful 

 lateral movements of the tail in swimming. Having in mind the desirability of 

 determining the nature of the rib articulations in the pelvic region of Mixo- 

 saurns, the writer has examined a number of the excellent specimens at Milan. 

 This examination shows that in some cases, at least, the short ribs near the 

 pelvic region or the base of the tail have broad, undivided heads, as in the 

 shastasaurian forms of the American Trias. In other instances ribs belonging 



