52 REPORT— 1839. 



The head of the Plesiosauri resembles that of the Crocodiles 

 in its general form, but is relatively much smaller in proportion 

 to the body : the elongated form of the strong and prominent 

 cranial bones, most of which extend from point to point, with 

 wide interspaces like the timbers of a scaffolding, forms one of 

 the first indications of a deviation from the Crocodilian type, 

 and of the affinity of the Plesiosaurus to the Lacertian Sauria ; 

 and this affinity is further exemplified in the condition of many 

 of the individual bones. 



The occipital bone includes the basilar, lateral or ex-occipital 

 aiid supra-occipital pieces in a permanently separated condition, 

 as in other Reptiles. The basi-occipital forms a larger propor- 

 tion of the articular tubercle for the atlas, and the ex-occipitals 

 a less proportion, than in the Crocodiles ; and the circumfe- 

 rence of the foramen magnum is completed by the supra- 

 occipital element ; in both which respects the Plesiosaurus 

 manifests its affinity with the Lacertian Sauria. 



The mastoid elements extend from the occipital to the tym- 

 panic bones ; but above these and between the occiput and the 

 strong arched pedicle supporting the lower jaw there is a vacuity 

 leading from the occipital region into the temporal fossae. The 

 corresponding openings in the skull of the Crocodiles are re- 

 duced to very small size in consequence of the expanded form 

 and oblique position of the tympanic bone, but in the Lacertian 

 Sauria they are as wide as, if not wider than, in the Plesiosaurus. 



The parietal is a strong triradiate bone in the Plesiosaurus, 

 consisting of a median piece corresponding with the normal 

 parietal in the Crocodiles, and of two transverse elongated pro- 

 cesses, formed, as it were, by a bifurcation of the posterior part 

 of the median piece. 



In a young specimen of Plesiosaurus macrocephalus in the 

 collection of Viscount Cole, the median or sagittal suture dividing 

 the two parietals is still distinct : in older specimens of the PI. 

 Hawkinsii I have always found it obliterated, so as to justify 

 the above description of the parietal as a single triradiate bone. 



The median portion of the parietal offers two modifications of 

 structure Avhich point out in a striking manner the deviation of 

 the Plesiosaurus from the Crocodilian, and its approximation 

 to the Lacertian type of the Saurian structure. 



The first of these characters is the median crest or ridge 

 from which the surface slopes away on each side ; proving that 

 the temporal muscles were relatively as strongly developed as 

 in the Iguance, e. g., and were only separated from one another 

 at the top of the head by the intermuscular ridge. In the Cro- 

 codiles on the contrary, in which the ponderous jaws are worked 



