XIII. 



PLEIOSAURUS MACROMERUS. 



355 



These cervical vertebrae are remarkable for the clear division of 

 the boldly-marked cicatrices into two nearly equal parts, so as to 

 fit a large divided head of a rib. They are all somewhat short, 

 nearly circular, roughly striated on the surface, and have a slight 

 rising on the anterior edge of the lower surface, which in one 

 specimen becomes a distinct boss. 



The articular faces are slightly concave, some swelling up round 

 the middle, which is commonly pitted ; the edge is but little re- 

 volute, neatly bordered (mostly by a marginal groove). The circular 

 sweep of the face is subtruncate above by the amplitude of the 

 base of the neurapophysis. 



Length of the largest, 2*8 inches ; breadth, 5-65 (over the apo- 

 physes, 6-8); height to canal, 5-2. Length of the smallest, 1-9; 

 breadth, 3-3 (over the apophyses, 4-1) ; height to canal, 3-0. 



Two foramina appear in the floor of the neural canal, and usually 

 one or two are observed on the sides. 



Of posterior cervicals (rather than anterior dorsals) we have only 

 a few specimens, characterized by large oval bases of the ribs, which 

 are slightly emarginate, and thus gradually pass to the others, which 

 have the cicatrix quite entire. In a fine specimen from Sandford 

 the articular face is undulated ; the centre usually rises in a large 

 umbo, round which the disk is concave. The lower surface rises 

 to a prominent tuberosity on the anterior edge. 



Diagram CXLIX. Posterior cervical vertebra of Pleiosaurus macromerus. 



Scale one-fifth of nature. 

 i. Cervical vertebra, seen in front. 2. The same, seen on the left side. 



Length, 3-3 inches ; breadth of articular face, 5-4 ; height, 5-5. 

 The length of this vertebra is greatest in the lower part. 



One dorsal vertebra from the railway near Swindon is of a 



A a 2 



