SAUROPTERYGIA 229 



ment, and form, with those of the fore limb. The lemur has 

 the hind margin less concave, and so appears more straight. 

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

 ulna. The tarsal hones 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 palate-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 strongly manifested ; but to this vertebral 

 character such affinity is limited ; all the others exemplify 

 the ordinal distinction of the Plesiosaurs from known existing 

 reptiles. The shape of the joints of the centrums ; the num- 

 ber of vertebra? between the head and tail, especially of those 

 of the neck ; the slight indication of the sacral vertebrae ; the 

 non-confluence of the caudal haemapophyses with each other, 

 arc all " plesiosauroid." In the size and number of abdominal 



