118 MEMOIRS OF THE UNIVERSITY OF CALIFORNIA. 



Ribs. Excepting on the anterior six to twelve vertebrae, the ribs of C. pe- 

 trinus are all single headed (pi. 10). The short double-headed ribs have clearly 

 notched heads, and articulate on distinctly separated diapophyses and parapo- 

 physes. As in Shastasaurus, in passing back through the vertebral series, the 

 lateral apophyses of the vertebral centra remain widely separated to the point 

 of disappearance of the parapophysis. If, as has been suggested by Yakowlew, 

 the bicipital articulation was being developed in these forms through division 

 of the rib head and of the vertebral apophysis with which it was in articulation, 

 one would expect in passing forward to the region where separate apophyses 

 actually exist to find the parapophyses gradually separating from the diapo- 

 physes. The fact that this is not the case may justly be urged against the view 

 that the primitive ichthyosaurian possessed single-headed ribs. If a double 

 articulation has been introduced in the American forms it must have appeared 

 through the development of a parapophysis sitiiated a considerable distance 

 away from the diapophysis. If the double articulation extended from the 

 cervical region oiit to the dorsals, the progress was most probably throiigh the 

 formation along the line of advance of rudimentary apophyses with no func- 

 tion of articulation. As yet no reason has been given why independent pro- 

 cesses should grow out from the region of the paropophysis and the lower head 

 to meet and form an inferior articulation. In addition to the arguments that 

 have been presented, it is to be remembered that some of the oldest known 

 ichthyosaurian forms, viz., members of the M.(?) ataviis group, have distinct 

 double articulations in the posterior dorsal region. 



The ribs of the dorsal region of Cymbospondylus petrinus are rather heavy, 

 but have in general the form seen in Ichthyosaurus. The heads are thick in- 

 feriorly, and the superior portion of the articular face curves away from the 

 upper part of the diapophysis, evidently leaving a gap filled by cartilage as 

 in Shastasaurus. The posterior dorsal ribs have remarkably wide heads, but 

 nowhere has there been observed a tendency for the articulating surface to 

 divide into two faces, as has been suggested in the case of Mixosaurus. 



In specimen 9950 short caudal ribs are present with the anterior caudal ver- 

 tebrae. As shown in plate 10, figure 7, they have nearly round articular heads, 

 but are considerably broadened distally. 



Abdominal ribs are found in considerable numbers with most of the larger 

 specimens, but are usually too much scattered about to show definitely their 

 relations to each other. With specimen 9154 there is preserved a consider- 

 able section of the abdominal plastron in which the elements have suffered no 

 disturbance (pi. 10, fig. 8). As nearly as can be determined there are usually 

 two transverse series of abdominal ribs for every pair of true ribs. Each 

 transverse series consists of a V-shaped median piece and two lateral pieces. 

 The median elements are heavy, and are only slightly flattened anteropost- 



