Structure of the Plesiosaurus Macrocephalus. 535 



correspond with that tooth, and offers a resemblance to a very characteristic 

 structure in the true Crocodiles. The lachrymal bone forms a great pro- 

 portion of the anterior and inferior part of the orbit: the superior maxillary 

 appears to form a small part of the orbital circumference below the lachrymal, 

 and the malar bone rests by an oblique suture upon its posterior extremity ; 

 the posterior margin of the malar is joined to the posterior frontal, thus com- 

 pleting the posterior osseous boundary of the orbit, and to the zygomatic, or 

 temporal bone. 



The concave articular surface of the lower jaw is well displayed in Lord 

 Cole's specimen, indicating free and frequent motion of that bone; the poste- 

 rior process of the angle of the jaw closely resembles that of the Crocodile. 

 The suture between the angular and supra-angular or coronoid process is 

 well displayed, showing the same complicated structure of the lower jaw 

 which Messrs. Conybeare and De la Beche have so admirably illustrated in the 

 allied genus Ichthyosaurus. The coronoid processes, however, rise higher, 

 and form a sharper edge for the insertion of the temporal muscles than in the 

 Crocodiles; and we have another important indication of affinity to the La- 

 certine, at the expense of departure from the Crocodilian type, in the absence 

 of the vacuity between the angular and supra-angular pieces, which vacuity 

 peculiarly characterizes the Crocodiles. 



The correspondence in the position of the head in the present specimen 

 with that of one of Mr. Hawkins's beautiful specimens of the species which I 

 have dedicated to him, and which he has obligingly permitted me to deli- 

 neate (PI. XLV.), enables me to state that, in the PL Macrocephalus, the 

 maxillary portion of the cranium is relatively larger, as compared with the 

 cranial portion, than in the PL Hawkinsii ; that the orbit is nearer the back 

 part of the cranium, and the temporal fossa consequently shorter; and that 

 the tympanic arches, upon which play the branches of the lower jaw, are 

 relatively much stronger. 



In all these points, as also in the relative size of the head to the trunk, and 

 in the shorter but stronger neck, we perceive the most admirable illustrations 

 of the laws of organic co-existence ; and at the same time are enabled to trace 

 a closer affinity, in the present species, than in any other hitherto described, 

 to the Crocodilian and Ichthyosaurian types of structure. 



