plesiosauridj;. 253 



A. Macrospondyline Group. 



Skull (when known) small, with short mandibular symphysis. 

 Cervical vertebrae (fig. 78) with the centra much elongated, with 

 elliptical and more or less flattened terminal faces and double 

 costal facets. Caudals elongated. Pectoral girdle unknown. 



The close resemblance between the centra of the cervical vertebrae 

 of this group and those of the typical group of Cimoliosaurus suggests 

 the descent of the latter from the present group. 



Plesiosaurus homalospondylus, Owen 1 . 



Of medium size. Skull veiy small, of nearly equal width through- 

 out, the premaxillae being very broad : neck very long, the length 

 of the skull being contained 6| times in the interval between the 

 atlas and the pectoral girdle. About 38 cervical vertebrae, in which 

 the centra are very broad and flattened inferiorly, with the costal 

 facets much elongated antero-posteriorly, while the neural spines 

 are extremely tall and wide. Six bones in carpus and tarsus ; 

 radius and ulna elongated. 



Hah. Europe (England and France). 



36184. Slab showing the nearly entire skeleton ; from the Upper 

 {Fig.) Lias of Whitby, Yorkshire. The type specimen ; described 

 and figured by Owen in his ' Liassic Reptilia,' Sauro- 

 pterygia, p. 12, pis. v.-vii. With the exception of part of 

 the caudal region, the entire vertebral column is shown ; in 

 the cervical region from the dorsal, and posteriorly from 

 the right lateral aspect. Of the limbs only the propodial 

 bones are shown ; and only a moiety of the right side of 

 the pectoral girdle is exhibited. A radius and a tibia, 

 as well as several of the more distal bones of the paddles 

 are preserved separately. Purchased, 1861. 



36184 a. Casts of the 13th and 14th cervical vertebrae of the pre- 

 ceding specimen. The dimensions of the larger centrum 

 are:— length 0,049 (1-93 inches), height 0,034 (1-34 

 inches), width 0,048 (1-88 inches). The fourteenth is 

 figured by Owen, op. cit. pi. v. figs. 2-4. 



Made in the Museum. 



R. 1334. Two later contiguous imperfect cervical vertebrae ; from 

 the Upper Lias of Whitby. The neural spines and ribs 

 are wanting; and the double costal facets are very dearly 



1 Liassic Ecptilia (Mon. Pal. Soc.)— Sauroplcrygin, p. 12 (18(55). 



