SAUROPTEKYGIA. 249 



margin less concave, and so appears more straight. The 

 fibula, in its reniform shape, agrees with its homotype the 

 ulna. The tarsal bones are also smallest on the tibial side. 

 Of existing reptiles, the lizards, and amongst these the old 

 world Monitors (Varanus, Fitz.), by reason of the cranial 

 vacuities in front of the orbits, most resemble the Plesiosaur 

 in the structure of the skull. The division of the nostrils, the 

 vacuities in the occipital region between the exoccipitals and 

 tympanies, the parietal foramen, the zygomatic extension of 

 the post-frontal, the palato-maxillary, and pterygo-sphenoid 

 vacuities in the bony palate, are all lacertian characters, as 

 contradistinguished from crocodilian ones. 



But the antorbital vacuities between the nasal, pre-frontal, 

 and maxillary bones are the sole external nostrils in the 

 Plesiosaurs; the zygomatic arch abuts against the fore part 

 of the tympanic, and fixes it. A much greater extent of the 

 roof of the mouth is ossified than in lizards, and the palato- 

 maxillary and pterygo-sphenoid fissures are reduced to small 

 size. The teeth, finally, are implanted in distinct sockets. 

 That the Plesiosaur had the "head of a lizard" is an emphatic 

 mode of expressing the amount of resemblance in their cranial 

 conformation. The crocodilian affinities, however, are not 

 confined to the teeth, but extend to the structure of the skull 

 itself. 



In the simple mode of articulation of the ribs the lacertian 

 affinity is again manifested ; the other vertebral characters 

 exemplify the ordinal distinction of the Plesiosaurs from known 

 existing reptiles. The shape of the joints of the centrums ; 

 the number of vertebrae between the head and tail, especially 

 of those of the neck ; the slight indication of the sacral verte- 

 brae ; the non-confluence of the caudal haemapophyses with 

 each other, are all " plesiosauroid." In the size and number 

 of abdominal ribs and sternum may perhaps be discerned a 

 first step in that series of development of the haeraapophyses 



