123 



The dorsals liave no liy[)ai)opliy.sis, and llic diapopliyses decnsasc in ver- 

 tical extent toward the posterior part of the scries. The greatest variation 

 is presented by the different species in the long series preceding the caudals, 

 which do not present zygapophyscs. The posterior of this series are much 

 shorter than the anterior; the former having the form of the dorsals, the lat- 

 ter of the caudals. In the Mosasaurus dekayi, M. gracilis, and M. giganteus, 

 none of these are depressed ; the shorter are subpentagonal in section. In 

 M. deprcssus and 3f. missuncnsis, the longer are depressed, while the dcprc-.s- 

 sion of the shorter diminishes I'egularly to the distal caudal series. In M. 

 hrumhyi, the long vertebrce are flattened to a still greater degree. (Sec 

 Gibbes' Monograph on Mosasaurus and its allies.) 



The caudals are divided into three series by Cuvier, viz : those with sep- 

 arate chevron-bones; those where the latter arc united to the centrum; and 

 those without them. Passing posteriorly, these vertebrae become gradually 

 shorter and more vertically ovate in form. The more posterior are less nar- 

 rowed in the M. maximus, M. oarthrus, and M. missuriensis {vide Leidy's 

 work) ; while, in M. dekayi and the Liodons, they are rather more narrowi'd 

 vertically. 



The characters of the diapophyses are marked in different parts of the 

 column. In all the species of the family, they descend from an elevated posi- 

 tion on the cervicals and anterior dorsals to an inferior one on the lunibars. 

 They never spring from the neural arch, as in the Archosauria, but always 

 from the base of it. On the median dorsals, they originate from the middle 

 of the side of the centrum, and, on the lumbo-sacrals, from the i)lane of the 

 inferior surface. They diminish in size, and, as soon as the articulations of 

 the chevron-bones appear, begin to ascend again. Ou the anterior caudals, 

 they rise to near the middle of the centrum, and gradually disappear at difi'er- 

 ent points in the different species. 



The chevron-bones are free throughout the anterior part of the caudal 

 series in the M. giganteus, and confluent with the centrum in the posterior 

 portions. This is probably the case with many species of the genus. During 

 immaturity, they may be all distinct in Mosasaurus, while in the genus Liodon 

 (vcl Macrosaurus) this condition is permanent throughout life, and thus 

 characteristic. Thoy have two short ha'mapophysial limbs, and a very long, 

 tapering spine, which is grooved in front. On the middle caudal of certain 



