212 THE ANATOMY OF VERTEBRATED ANIMALS. 



each end — and of sis otliei* bones, three on each side, which 

 are elongated and pointed at each end, and so disposed 

 that their pointed ends overlap one another. (Fig. 68, C.) 



In some Plesiosauria, 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 fossae, being wide. The 

 occipital condyle 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 parie- 

 tals send off comparatively short processes backwards, which 

 become connected with the large squamosals. The latter 

 unite with the postfrontals, which separate the orbits from 

 the temporal fossa, and the orbit is completed behind by 

 the junction of the postfrontal with the jugal. The jugal 

 bone is continued backwards 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 circum- 

 stance in which the skull of Plesiosuurus differs from that 

 of most Reptilia 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 i^tei-ygoid bones, which unite in 

 the middle line, and send processes outwards and backwards 

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

 this region of the skull, is seen an ovoidal fossa or depres- 

 sion. The pterygoids are continued forwards, and are 

 united externally with transverse bones, and more an- 

 teriorly with flattened palatine bones. When the forepart 

 of the under-surface of the skull is exposed, two other fossae 

 are visible, one on each side of the middle line, bounded 



