46- MARINE EEPTILES OF THE OXEORD CLAY. 



(text-fig. 27, A, B), and finally disappear altogether a little in front of the point where 

 the vertebral column bends downwards. 



The ventral ribs seem to have been very slender and slightly developed, but they are 

 not sufficiently known for description. 



Shoulder-girdle and Fore Limb. — The shoulder-girdle of O^hthalmosaurus has already 

 been described in some detail by Prof. Seeley *, and, in fact, it is upon the characters of 

 this part of the skeleton that the genus is founded. Unfortunately, however, the 

 coracoids and one of the scapulae of the type specimen are diseased and altogether 

 abnormal, the left scapula, in fact, being an almost shapeless mass of bone, with which 

 the upper end of the left clavicle is fused: probably this abnormality is due to 

 extensive injuries received when the animal was comparatively small. The left 

 coracoid, figured by Prof. Seeley, has a deep notch in its posterior border not found in 

 normal specimens : this peculiarity led to some errors in the original account subse- 

 quently corrected by the author f. Later descriptions were founded on a nearly 

 perfect shoulder-girdle, which has been figured by Prof. Seeley % and forms the basis 

 of the present account. The closely similar shoulder-girdle of Ba^ptanodon discus has 

 been figured and described by Gilmore §. 



The coracoids (text-fig. 32) are broad plates of bone thickening greatly towards 

 their inner edge and to a less degree towards the outer edge. The inner thickening 

 (text-fig. 32, C) terminates in a rugose oval surface, placed somewhat obUquely, for 

 union with the opposite member of the pair, there being a pad of cartilage between 

 the two, except perhaps in very old individuals. The upper (visceral) surface is 

 concave from side to side, but the anterior half, owing to the thickening of that 

 region, is convex from before backwards, while the comparatively thin posterior 

 portion is concave in the same direction. The ventral surface is, speaking generally, 

 gently convex from before backwards and concave from side to side. The thin 

 posterior border is evenly convex and in life was fringed with cartilage, as is shown 

 by its irregularly grooved edge. The outer border is occupied mainly by the large, 

 nearly flat glenoid surface, which in life was no doubt covered with cartilage. It widens 

 out from behind forwards, and anteriorly is coterminous with the facet for union with 

 the scapula: this surface makes an angle of about 135° with the glenoid facet; it is 

 nearly triangular and was covered with cartilage. The outer half of the anterior 

 coracoid border, immediately internal to the scapular facet, is thin and sharp-edged ; 

 it is deeply concave, forming a sharply-defined notch or bay. This notch seems to 

 represent the remains of the sharp anterior border of the originally much more 



* Quai't. Journ. G-eol. Soc. vol. xxx. (1874) p. 696, pis. xlv., xlvi. figs. 1, 2 ; also Proc. Eoy. Soc. vol. liv. 

 OS93-4) p. 149. 



t Proc. Roy. Soc. torn. cit. p. 151. 



H Loc. cit. 



§ " Osteology of Baptanodon" Mem. Carnegie Museum, vol. ii. pp. 108-113, test-figs. 21-22, pi. xii. 



