534 Richard Owen, Esq ,, on the 



From what can be discerned of the structure of the posterior part of the 

 skull in the present specimen, and from the appearance of the Has-matrix 

 which has intruded beneath the tympanic arch, on the right side, into the 

 temporal fossae, there is sufficient evidence that, in the PL Macrocephalus, 

 a large opening on each side of the occipital region of the cranium leads be- 

 neath the parieto-tympanic arches into the temporal fossae. This conforma- 

 tion differs from that which is expressed in the posterior view of the restored 

 head of the Plesiosaur, given by Mr. Hawkins in the 23rd Plate of his Me- 

 moir; but repeated and close examination of the skull, both of Lord Cole's 

 specimen and of the PL Hawkinsii, prove that the Plesiosaurus participated 

 in the Lacertine type of cranial structure and deviated from the Crocodilian*, 

 in the presence of the openings above described. 



When we come to consider the facial and maxillary bones, the correspond- 

 ence with the Lacertine Sauria begins to diminish, and that with the Croco- 

 diles to increase. This tendency to the higher type of Saurian organization 

 is shown in the strength of the whole maxillary apparatus, in the great rela- 

 tive size of the intermaxillaries, the roughened exterior surface, and the 

 distinct alveolar cavities for the teeth. But the nostrils, in their size and po- 

 sition, combine with the structure of the paddles to indicate the affinity of the 

 extinct Enaliosaurs to the existing Cetaceans, and offer a beautiful example 

 of the adaptation of structure to the peculiar exigencies of a species. 



The apertures through which the air is respired are placed, in the PL Ma- 

 crocephalus, exactly where Mr. Conybeare has described them to be situated 

 in the Dolichodeirus, and where they are placed in the PL Hawkinsii, viz. a 

 little anterior to the orbits, near the highest part of the head. In the pre- 

 sent specimen it would seem that the intermaxillaries formed no part of the 

 boundary of the nasal apertures, while in the Crocodiles almost their en- 

 tire circumference is due to these bones. The nostrils in the Plesiosaur are 

 formed by an interspace at the convergence of the anterior frontal, nasal, and 

 superior maxillary bones. In the Lacertine Saurians, a small proportion at 

 least of the boundaries of the anterior nostrils is always formed by the inter- 

 maxillaries, and these orifices are separated by a bony longitudinal partition, 

 as in the Plesiosaur. The intermaxillary suture extends from the anterior 

 part of the nostril forwards to a little more than half-way between the orbit 

 and anterior extremity of the cranium. One of the strongest of the inferior 

 teeth rises just in front of this suture, and a slight notch at that part seems to 



♦ This communication is reduced to a very small opening in the Crocodiles in conseqnence of 

 the form and position of the os tyropanicum. 



