THE BONJiS IN THE ENALIOSAURIA. 



321 



The other bones of the extremities in Plesiosaurus\i-d.\e no avian 

 characters. 



§ 3. The Crocodilian Cliaracters of Plesiosanrus. 



In outline and in the depressed character, the skulls of typical 

 crocodiles and Plesiosaurs are similar. The nares, however, m 

 Plesiosaurs are far back on the skull, and only parted from the 

 orbit by the lachrymal bones, while in crocodiles they are single 

 and terminal. 



The orbits are not dissimilar in character, but in Plesiosaurs 

 they are in the middle of the skull, wbile in crocodiles they are 

 in the hinder third. The temporal fossae are much larger than 

 tbe orbits in Tlesiosattrus, while in crocodiles they are smaller. 

 These differences in the positions of organs and regions of the 

 skull necessitate proportionate differences in the length and. 

 form of the cranial bones. But in crocodiles the postfrontal is not 

 usually divided so as to exclude the frontal from the orbit ; and in 

 Plesiosaurs the postfrontal is divided from the squamosal by the 

 malar and quadrato-jugal, and thus the temporal fossa is enlarged. 

 The quadrate bone is similar in form, similarly placed in the skull, 

 except that in Plesiosaurs the pterygoid bone meets it on the 

 inner side; and usually it is similarly directed backward, ihe 

 occipital condyle is similarly formed in the two groups ; and the 

 occipital bones are not dissimilar. The palate is similarly closed, 

 except that in Plesiosaurs the posterior nares are not carried so 

 far back, being surrounded by the palatine, transverse, and pte- 

 rygoid bones, and that the transverse bones are more anterior, 

 and larger, and close the palatine foramina in front. The teeth 

 are exceedingly similar, and are similarly placed in sockets, m all 

 crocodiles except the Black Alligator. The lower jaw appears 

 to be similar in the proportions taken by the dentary, angular, 

 and opercular bones, in the form of the concave articulation, and 

 in the extent of the postarticular keel ; but in Plesiosaurs its side 

 is not perforated posteriorly. The cranial bones of Plesiosaurus, 

 though not always smooth, are never pitted as in crocodiles. 



In no known Plesiosaurus does the tail include so large a 

 proportion of vertebrae, or the neck so few as in crocodiles. In 

 Plesiosaurs the centrum is rarely, if ever, so long as in crocodiles ; 

 and the articular surfaces are never procoelous, but flat or slightly 

 concave at both ends. The neural arch is similar, except that, 

 ■like the ceutrimi. it is longer from front to back than in Plesio- 



