THK BONKS IK THE ENALIOSAUKIA. 319 



phalanges are so similar that it would not be possible to distin- 

 guish between them. They are compressed or rounded hourglass- 

 shaped bones witli flat or moderately convex articular ends which 

 often show numerous perforations for blood-vessels, around which 

 the bone is slightly elevated. The digits on the radial side have 

 most phalanges. There may be as few as three digits. 



§ 2. The Avian Gharacters of Plesiosaurus. 



The backward position and division of the anterior nares in 

 birds, and the consequent extension of the premaxillary bones and 

 backward position of the maxillary bones, accords well with the 

 condition of the prenasal part of the skull in Plesiosaurs. In them 

 the orbits are circumscribed by bones, being bordered below by 

 the malar ; they are placed much further forward, more towards 

 the middle of the skull, than in birds ; and thus there come to 

 be large circumscribed temporal fossae behind the orbits, to which 

 there is nothing similar in birds. The eyes are more horizontal 

 than in birds ; and some Plesiosaurs, like some birds, have a scle- 

 rotic circle of bones. Unlike birds, the quadrate bone is firmly 

 fixed in the skull, and covered, as in some struthious birds, by the 

 squamosal on the outside, which latter bone accordingly does not 

 enter into the brain-case. The cerebral part of the skull is of 

 altogether different form. The occipital articulation is similar, 

 being in both types contributed to by the exoccipital bones. 



The palate of struthious birds would be very similar to that of 

 Plesiosaurus if the palate were closed mesially and the posterior 

 nares carried through the infraorbital foramina, the nares closed 

 by the growing together of palatine and maxillary ; and it would 

 require that the pterygoid bones should be expanded inward and 

 backward, so as to meet behind the nares and cover the region 

 of the basisphenoid, carrying the quadrate bones backward with 

 them. The lower jaw in both is composite and moderately pro- 

 longed behind the concave articular groove for the distal end of the 

 quadrate bone. 



The vertebral column of Plesiosaurs only resembles that of 

 birds in the large number of vertebrae included in the neck. 

 The centrum differs in having the articular surfaces flat or concave 

 in Plesiosaurs. In birds the atlas and axis are not anchylosed, 

 the cervical vertebrae have no separate ribs, and the neural 

 spine is suppressed, while the vertebral artery is often carried 

 through a ring on the side of the centrum in birds as it is in some 



