THE PLESIOSAURIA. 213 



beliind by the palatine bones, and separated by what appear 

 to be the vomers. I conceive that these are the true pos- 

 terior nares, and that the posterior apertures are simply 

 spaces left between the pterygoid bones and the basis cranii. 



At the sides of the base of the skull, specimens of Ple- 

 siosaurus occasionally exhibit two styliform bones, which lie 

 parallel with the axis of the skull ; these may be parts of 

 the hyoidean apparatus. No trace of any sclerotic ring has 

 been found. 



The teeth of the Plesiosauria are sharp-pointed, curved, 

 and the outer surfaces of their crowns striated. Each 

 tooth is lodged in a distinct alveolus, with which, as in the 

 Crocodilia, it does not become ankylosed. 



The pectoral arch (Fig. 68, E) is one of the most remark- 

 able parts of the organization of the Plesiosauria. It con- 

 sists, in the first place, of two very large coracoids, the long 

 axes of which are parallel with one another, while their in- 

 ner edges meet, without overlapping, throughout the greater 

 part of their extent. In this respect they diifer from any 

 of the Lacertilia, which are provided with weU-developed 

 limbs. In these the long axes of the coracoids always cut 

 one another at a large angle, open posteriorly — a circum- 

 stance which results from the manner in which the cora- 

 coids are received into grooves in the anterolateral edges of 

 the rhomboidal part of the sternum. Hence it would 

 appear that the Plesiosauria, like the Chelonia, did not 

 possess anything corresponding to this rhomboidal part of 

 the sternum, but that the intercoracoid part of the sternum 

 was either absent, or reduced to a mere band, as in some 

 Batrachia. 



The scapulse are imlike the corresponding organs in any 

 other reptile. The glenoidal end, stout and strong, is 

 continued horizontally forwards and inwards, as a bony 

 prism, with a somewhat concave inner edge, and flat inferior 

 surface. The outer surface, rising up at right angles to 

 the ventral siarface, gives rise to a well-defined edge; at 

 a short distance from the glenoidal end, the part of the 

 bone which bears this surface is produced upwards and 



