Mbrriam.] 



Triassic IcMJiyopterygia . 



71 



where the ribs are lower down on the side of the centrum, the 

 pedicels lose their lateral projections. 



On all excepting the posterior caudals, the arches are held 

 together by well developed zygapophyses. In the cervicals they 

 are large, strong, and wide. In the posterior dorsals they are 

 weak but still effective, and are placed a little higher up oil the 

 spines. In all eases the facets are paired and separated by a 

 narrow depression. They tend, however, to approach the same 

 plane. In osmonti (PI. !), fig. 8) those in the anterior portion 

 of the column if extended would ordinarily intersect at an angle 

 between 150° and 160°, while in the middle or posterior dorsal 

 region they practically come into the same plane. Their surfaces 

 are slightly roughened, indicating the presence of considerable 

 cartilage. They seem to be about intermediate in form between 

 the type found in Mixosaurus, in which the facets cut each other 

 at a fairly sharp angle, and that of the later Ichthyosaurs with 

 the facets reduced, brought into the same plane, and united. The 

 primitive form of zygapophysis is retained longest in the cervical 

 and anterior dorsal vertebrae, or in those which have been least 

 affected in the development of the powerful sculling tail of the 

 Ichy thyopterygia . 



The mode of articulation of the dorsal ribs with the vertebrae 

 furnishes one of the most important characters of this genus. In 

 the anterior nine or ten vertebra? of osmonti (PI. 8) and in those 

 immediately behind the skull of alexandrce (PI. 12), two distinct 

 prominences with articular surfaces for the reception of the 

 ribs are present. On the first and second vertebra? present in 

 osmonti, the parapophysis has about half the vertical diameter 

 of the diapophysis and is slightly anterior to it. Farther back 

 on the column the parapophyses gradually decrease in size and 

 were certainly not functional beyond the ninth or tenth. On the 

 eleventh, there is a mere tubercle without an articular surface 

 for the rib. The last trace is seen on the seventeenth, where a 

 minute rudiment is present on one side only. In alexandrce 

 the parapophysis of the fourth vertebra behind the skull is consid- 

 erably less than half the length of the diapophysis, and the 

 parapophysis of the tenth is so small that it was probably not 

 functional. 



