98 MEMOIRS OP THE UNIVERSITY OF CALIFORNIA. 



for the spinal canal is relatively much larger than in the large unquestionable 

 neural arches figured by Repossi (1902, tav. 9, fig. 9&). The lower arches 

 extend behind the bend in the tail and appear to be at least as long in that region 

 as are the considerably shortened neural arches. 



The nature of the inferior arches below the region in the tail in which the 

 upper arches are elongated most is not clear. According to Repossi 's repre- 

 sentation of this region (1902, tav. 9, fig. 8), the facets for chevrons are much 

 reduced or absent here. The inferior arches sketched in dotted lines in Re- 

 possi 's figure do not actually exist in the specimen. On another specimen in 

 which the caudal region is otherwise well shown, the hypocentra are again 

 absent from this region. Though lower arches arc present farther back in the 

 tail, it is possible that they became reduced below the region of upward curv- 

 ature. 



Ribs. In contrast to the bi capital articulation in the typical ichthyosaurs, 

 the heavy ribs of the dorsal region articulate on a single head (see fig. 60^, p. 

 49), as has been shown by Repossi. In the cervical region the rib articula- 

 tion seems to be single-headed as in the dorsal region. On one specimen a 

 short rib, apparently belonging to the third or fourth vertebra behind the skull, 

 has a single head. As interpreted and figured by Repossi, the ribs of the pos- 

 terior dorsal or lumbar' vertebrae are double-headed. 40 In discussing this pecul- 

 iarity of Mixosaurus, Yakowlew (1902. 1 and 2) has suggested that as the single- 

 headed rib precedes the double-headed in the history of the Ichthyosauria, the 

 latter form is evidently to be derived from a single articulation by some pro- 

 cess of division of the rib head. Since this division seemed to appear first in 

 the pelvic region of Mixosaurus the suggestion was made that its appearance 

 was connected with the extraordinary movements of this portion of the body in 

 the use of the tail as a sculling organ. The relations of the ribs of Mixosaurus 

 as described being entirely different from those of the American forms, in 

 which a double articulation occurs in the neck only; the writer has examined, 

 as carefully as possible, the excellent specimens on exhibition in the Milan 

 museum, for evidence bearing on this point. 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 to the anterior caudal region seem 

 actually to show r a beginning division of the articular face. In one case where 

 the evidence seemed strongest, the apophysis on the vertebral centrum showed 

 also some indication of division. It should, however, be noted that even in 

 this instance the evidence hardly appears to warrant a definite statement that 

 double articulation occurs, as the side of the vertebral centrum is badly broken 

 up, and does not present a satisfactory case. The heads of these ribs are, 



* Through a typographical error the original descriptions did not agree with the figures. 



