THE PLESIOSAURIA. 183 



pointed at each end, and so disposed that their pointed ends 

 overlap one another. (Fig. 68, C.) 



In some JPlesiosauria, as already stated, the skull (Fig. 68, 

 A) is very small in proportion to the body, not having more 

 than a twelfth, or a thirteenth, of the length of the latter ; but, 

 in other species, the skull is much larger. The snout is taper- 

 ing and depressed, and the nasal apertures are situated, not 

 at its extremities, but just in front of the orbits — the latter, 

 like the supra-temporal fossse, being wide. The occipital con- 

 dyle is almost wholly developed from the stout basi-occipital. 

 The ex-occipitals give off elongated parotic processes, and the 

 basisphenoid is a thick bone, which ends in front in a long 

 rostrum. 



There is a well-marked parietal foramen, and the parietals 

 send off comparatively short processes backward, which be- 

 come connected with the large squamosals. The latter unite 

 with the postfrontals, which separate the orbits from the tem- 

 poral fossa, and the orbit is completed behind by the junction 

 of the postfrontal with the jugal. The jugal bone is continued 

 backward into a slender bar, which extends as far back as the 

 lower end of the quadrate, and probably contains a quadrato- 

 jugal, so that* there is a distinct infra-temporal fossa. The 

 most obvious circumstance in which the skull of Plesiosaurus 

 diifers from that of most Iteptilia is in the great size of the 

 premaxillaries, which constitute a large proportion of the 

 snout. 



The under-surface of the skull is rarely well exposed in 

 its anterior part ; posteriorly, it exhibits a broad and long 

 expansion, formed by the pterygoid bones, which unite in the 

 middle line, and send processes outward and backward to 

 the quadrate bone. On each side of the middle line of this 

 region of the skull, is seen an ovoidal fossa or depression. 

 The pterygoids are continued forward, and are united exter- 

 nally with transverse bones, and more anteriorly with flattened 

 palatine bones. When the fore-part of the under-surface of the 

 skull is exposed, two other fossae are visible, one on each side 

 of the middle line, bounded behind by the palatine bones, and 

 separated by what appear to be the vomers. I conceive that 

 these are the true posterior nares, and that the posterior aper- 

 tures are simply spaces left between the pterygoid bones and 

 the basis cranii. 



At the sides of the base of the skuU, specimens of Plesio- 

 saurus occasionally exhibit two styliform bones, which lie 

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



