BRITISH FOSSIL REITILKS. 53 



principally by the masseteric and pterygoid muscles, the tempo- 

 rals are small, and are separated from each other by a flattened 

 space occupying nearly the whole of the simple parietal bone. 



The second character of the median part of the parietal, which 

 brings the Plesiosaurus near to the Lizard tribe, is a moderate- 

 sized elliptical vertical perforation of the bone, a few lines be- 

 hind the coronal suture, which perforation is analogous to that 

 described by the Rev. Lansdowne Guilding in the Iguana 

 under the name of the Foramen Homianum, where, however, 

 it is situated directly upon the coronal suture, in the situation 

 of the anterior font ane lie. The same foramen, however, exists 

 in many other genera of Lacertian Sauria ; and in Mo7iitor, La- 

 certa proper, &c. it is situated, as in the Plesiosaurus, entirely 

 in the parietal bone. There is no trace of this foramen in th.e 

 Crocodilian Sauria. The posterior bifurcation of the parietal 

 bone forms a third instance of the resemblance of the Plesio- 

 saurus with the Lacertian, and its difference from the Croco- 

 dilian structure. These processes are of considerable strength, 

 and commonly form the most prominent parts of the cranium in 

 fossil specimens : they articulate by means of an oblique sigmoid 

 suture with the tympanic bone*. 



Frontal. — The frontal bone consists of a median, two anterior 

 and two posterior pieces. The median frontals extend as far 

 forwards as the midspace between the small nostrils, and appear 

 to terminate in a point between the commencement of the narrow 

 nasal bones. The interfrontal suture in the young PL rnacro- 

 cephalus before alluded to, is elevated by a ridge continued for- 

 wards from the parietal crest. The outer margin of the median 

 frontal forms the superior boundary of the orbit. The anterior 

 frontal enters into the formation of the anterior and superior 

 angle of the orbit, and is wedged in between the mid-frontal 

 and superior maxillary bones. The posterior frontal bounds 

 tiie orbit posteriori}^, and extends downwards to join the malar 

 bone, like the columnar portion of the post-frontal bone in the 

 Crocodiles ; but it is broader and more superficially situated in 

 the Plesiosaurus, and thus resembles more the corresponding 

 part of the cranial structure in the Lacertian Sauria. The pos- 

 terior frontal differs further and in a more striking degree from 

 the Crocodilian type in not being extended backwards to join the* 

 mastoid ; so that the skull of the Plesiosaurus does not present, 



* One of Miss Philpott's specimens exhibits the parietal of a PI. doVicJioJeirns 

 thinned oft' posteriorly, and rugous, apparently forming an articulating or sutu- 

 ral surface for the overlapping of the tympanic bone. 



In most specimens the sagittal suture, dividing the median parietals, is per- 

 sistent. 



