the Shoulder-girdle of a Plesiosaur. 335 



but thin posteriorly ; their hinder outer aug-les are pro- 

 longed into short thick rod-like projections. The humerus 

 when fully grown is immensely expanded at its distal end. 

 The radius is very large and vertically elongate ; the ulna 

 very wide transversely. On the postaxial side of tliis latter 

 there is usually a small ossification, which may fuse with it 

 or with the ulnare, and in one specimen is in contact with 

 the humerus ; this bone may perhaps be regarded as a 

 pisiform. In the pelvis the pubis and ischium met in the 

 middle line in the adult, closing the obturator foramen. Tlie 

 ilium does not touch the pubis, a character, however, which 

 seems to be universal among the Plesiosaurs. The femur 

 is not nearly so much expanded distally as the humerus. 



In a future paper I hope to give a complete account of 

 some of the more important osteological characters of this 

 genus. 



With regard to the specific name of this reptile there is some 

 difficulty. Examination of a considerable number of more or 

 less complete sets of associated bones shows that there are 

 two forms possessing the generic characters given above and 

 differing only in size when compared at corresponding ages as 

 determined by the condition of the scapulae, cervical ribs, &c. 

 Mr. Leeds is of opinion that this may indicate the occurrence 

 of sexual dimorphism in these Plesiosaurs, the larger bones, 

 which are always proportionately more massive and possess 

 more strongly marked surfaces for muscle-attachment, being 

 those of males, while the smaller, more lightly built skeletons 

 are those of females. Such sexual dimorphism does occur in 

 reptiles; for example, it is well-marked in the case of 

 Spheuodon, a circumstance of some interest, since that reptile 

 shows some affinities with the Sauropterygia. At present, 

 therefore, it seems best to refer all these remains to a single 

 species, the name of which will be Cryptoclidus oxoniensis, 

 this specific natue having been applied by Phillips * to 

 cervical vertebras which differ only in size from tho.'^e of 

 Cryptoclidus platymerus^ Seeley. Moreover, Phillips rightly 

 referred to the same species scapulae (described as ischia), 

 dorsal vertebras, and other bones which resemble those of 

 the type specimen G. platymerus, but belonged to a younger 

 individual. 



Turning now to the descriptionof the shoulder-girdle (fig. 1), 

 and taking the adult condition first, we find that the scapula is 

 of the usual triradiateform, consisting of a backwardly directed 



* ' Geology of Oxford,' 1371, p. 307. 



23* 



