32 .MK.MOIRS OP THE UNIVERSITY OF CALIFORNIA. 



excepting the doubtful vertebrae described by von Meyer 

 (Palaeontg. B. 1, p. 253). In the earliest forms from the Middle Triassic the 

 centra are generally deeply biconcave, the concavity beginning at or close to the 

 margin of the centrum. In Mixosrturus (fig. 121) a slight marginal flattening 

 may be present, but it is not comparable to that of C ymbospondyliis. As yet 

 the extreme flattening in Cymbospondylus has been observed only on specimens 

 of the large C. petrinus. In the type of C. piscosus (fig. 134) the concavity of 

 the centra is like that commonly seen in Ichthyosaurus, so that large size 

 may be an important factor in this case. On the other hand the enormous cen- 

 tra of Shastasaurus careyi from the Upper Trias exhibit no filling out of 

 the intervertebral space. Among the more recent ichthyosaurians both perfor- 

 ation of the centrum and flattening of the peripheral portion of the terminal 

 faces are occasionally seen, and appear together in Ichthyosaurus campylodon. 

 The proportions of the vertebral centra in the different regions of the col- 

 umn are in general much as in the ichthyosaurs of the Jurassic. The centra 

 are generally broader than high in the cervical region, and gradually increase in 

 height to the middle dorsal region, where the vertical and transverse diameters 

 are nearly equal (figs. 18-20). Behind this point the transverse diameter be- 



comes relatively small until in the posterior caudal re- 

 gion of some forms the height may equal three o'r foul- 

 times the width. 



In the cervical and anterior dorsal regions the rib ar- 

 ticulation extends upward over the superior margin of 

 the centrum and covers a part of the base of the upper 

 arch (fig. 26). In the posterior dorsal region it moves 

 down to the lower margin, but rises again to about half 

 the height of the side of the centrum near the middle 

 26 caudal region. Particularly in Cymbospondylus petrinus 



Fig. 'J6. Delphinosaurux per- /_> ,i\ j_i -11 i IT 



rim Merriam. Antero- (fig. 24) the posterior dorsal centra take on a sharply 



triangular form where the rib articulations reach the 



vrtebra 

 x % lower margin of the centrum. 



The relative length of the anteroposterior diameter varies greatly among 

 the Triassic genera. In some, particularly the latest, largest and most general- 

 ly specialized forms the centra are relatively short and "high, as is exemplified 

 in Shastasaurus careyi (fig. 29), in which the length of a middle dorsal may 

 equal less than 29 per cent, of the height. In all of the earlier forms the centra 

 are relatively long. In Dclphinosaurus perrini (fig. 27), found principally in a 

 lower horizon than the forms with very short centra, the length of middle dor- 

 sal centra may equal 84 per cent of the height. The relatively great increase in 

 height and width of the dorsal centra is paralleled to a considerable extent in 

 the Cetacea. The extraordinary height of the centrum has been considered by 



