132 Prof. fi. G. Seeley. The Shoulder Girdle 



which is triangular, flat, very thin, and has perfectly straight sides, 

 which, in their hinder approximating two-thirds, are slighly bevelled. 

 There is no evidence given that the bone occupied the position which 

 has been figured, and I see no reason for believing that it was not 

 placed, as in other Sauropterygians, on the visceral surface of the 

 slightly inclined scapulae, where there is a doubtful indication of 

 what may be an imperfectly preserved right clavicle. If the straight 

 lateral border of the interclavicle was in contact with the flat visceral 

 surface of the scapula, the bones would be in harmonious relation. 

 The bevelled margin appears to look inward, and is therefore inferred 

 to have given attachment to a lateral ossification which was still 

 more delicately thin. This condition is shown in the following 

 figure of the bone. 



(iii.) A third modification of the Plesiosauriau type may be* in- 

 dicated by the specimen in the Leeds Collection in the British 

 Museum numbered 36. It is small, and the bones are not sharply 

 ossified and immature, as Mr. Leeds has always believed. But I 

 have not observed any specimen in his collection which would, with 

 certainty, represent its adult state. The bones of the shoulder 

 girdle are thick, and the scapula and coracoid are formed on the 

 Plesiosauriau type, in that the inner border of the scapula gives no 

 evidence of a median precoracoid prolongation backward to meet the 

 coracoid. There is no indication that the coracoids and scapulae 

 ever met in the median line, even in the supposed adult condition, 

 since there is no anterior median process to the coracoid ; but there is a 

 cartilaginous interval between them in front like that attributed to 

 Pliosaurus. The scapula is a stout triradiate bone with a wide external 

 process, and in form it resembles the bones attributed to Pliosaurus. 

 But the cervical vertebrae have no trace of the Pliosaurian modifica- 

 tion, and have the aspect of the vertebrae of Plesiosaurus, except that 

 the articulation for the rib is not divided in the cervical region. Some 

 Plesiosaurs from the Lias have shown the closest possible approxima- 

 tion of those surfaces, but the divided condition of the rib facet did not 

 terminate with the Lias species, since some specimens from the Wealden 

 (which are referred to Cimoliosaurus, 'Brit. Mus. Cat. Foss. Rept.,' 

 Part II, p. 227, No. 2,444, No. 26,000) retain the character in a condition 

 similar to that attributed to Thaumatosaurus carinatus (loc. cit., p. 168, 

 fig. 57). It may be that the imperfect ossification causes the facet of 

 bone to appear single in this Oxford Clay fossil, while its cartilaginous 

 terminations during life may have been divided ; but so far as the 

 evidence goes it rather suggests a sub-generic modification of the 

 genus Plesiosaurus as indicated by the scapular arch, distinguished by 

 undivided articular heads to the cervical ribs, if the adult preserved 



* I am not sure that this immature Plesiosaurian type did not, on attaining 

 maturity, become the Elasmosaurian genus CrypiGclidus. 



