56 Prof. R. Owen on the Occurrence 



short imperforate hsemapophysis, h. The series of these 

 hasmapophyses are homologues of those described by Sir 

 Philip de M. Grey Egerton under the name of " subver- 

 tebral wedge-bones" in Ichthyosaurus* . They are present in 

 similar intervertebral position in Arnblyrhynchus'f, where they 

 are five or six in number. They are represented by con- 

 fluent pieces in Cyclodus J. But the presence of hypapo- 

 physes for their support distinguishes the Mosasauroid from 

 other families of Lacertilia as well as from both Ichthyo- and 

 Sauropterygia. 



The diapophysis of the third cervical supports a rib ; and a 

 similar costigerous process § is present in the dorsal vertebras. 

 This series may be conveniently, though artificially, defined 

 by the suppression of the hypapophysis. The zygapophyses 

 (rigs. 5 and 6, z, z') disappear in the posterior dorsals as in 

 fig. 7. The diminution in vertical and increase in longi- 

 tudinal extent, together with its descent in position from the 

 side of the centrum, reduce the " transverse process " to a 

 " parapophysis," p, fig. 8, which characterizes the lumbar 

 vertebras. This change is attended with a modification of the 

 shape of the centrum, the transverse section of which becomes 

 triangular with the base downward, instead of the elliptic 

 shape shown in the vertebras from the antecedent part of the 

 column. There is no sacrum by ankylosis : a single ver- 

 tebra supports a pair of small rib-like, feebly curved, iliac 

 bones (ib. fig. 1, 1), with a slightly expanded, distal, bifaceted, 

 syndesmosal surface, the larger division for the ischium. The 

 pubics, u, have a smaller syndesmosal terminal expansion, are 

 slender and nearly straight. The ischia (m) are broader, have 

 a short posterior process, offer proximally a syndesmosal sur- 

 face divided between the ilium and the femur, and have a 

 distal surface for a symphysis with their fellow, completing 

 the inverted arch below. 



The anterior caudal vertebras add to the parapophyses 

 (fig. 9, p) a pair of hypapophyses, y y to which is articulated 

 a hasmal arch formed by the apical confluence of a pair of 

 hasmapophyses, h, from which confluence extends obliquely 

 backward a hasmal spine, h s, rivalling or surpassing the 

 neural one, n s, in length. The parapophysis gradually 

 shortens and disappears at about the anterior third of the tail 

 (fig. 10), which thereafter shows its natatory compression and 



* Trans. Geol. Soc. ser. 2, vol. v. p. 187, pi. 14 (1836). 

 + Ann. & Mag. Nat. Hist, id supra, fig. 4, p. 221. 

 X Ib. ib. fig. 5, p. 223. 



§ This, though marked d, combines the origins of both di- and par- 

 apophyses ; in fig. 8 it becomes p. 



