OSTEOLOGY. 



l 3 



only a slight notch; all the others have coalesct. The prezygapophyses are close 

 together, but have not coalesct. Those of the eighth are very small, thus greatly 

 different from those of the members of the other superfamilies. 



In some of the other Pleurodira the intercentrum and the odontoid process of 

 the first cervical are distinct from the arches. In the Pelomedusidse only the second 

 cervical is convexo-convex; all the others concavo-convex. 



In none of the Pleurodira is the neck withdrawn into the shell as it is in the other 

 turtles, but is bent sideways and brought under the projecting borders of the shell. 

 In harmony with this action, the anterior bones of the carapace and of the plastron 

 often project farther than in other turtles. 



The skull of the Pleurodires offers many peculiar structures. That of Hydrome- 

 ditsa may first be considered. This skull is long and very flat. There are distinct 

 nasal bones. The prefrontals do not send down processes to the vomer. There is a 

 postorbital arch, but no zygomatic arch. At the rear there is a very slender palato- 

 squamosal arch. Seen from above the whole upper surface of the pterygoids and the 

 whole temporal fossae are exposed to view. There is no ridge projecting outward 

 from the parietal over what may be called the suprapterygoid fossa, as in other 

 turtles; nor does the prootic project forward over this fossa. The fossa just men- 

 tioned is bounded outwardly by the upturned outer border of the broad pterygoids. 

 The premaxillae are small. The supraoccipital spine is extremely short. 



Fig. 6 represents the skull seen from below. The 

 triturating surfaces of the upper jaw are very narrow. 

 The choanal openings are very large and are separated 

 by the splint-like vomer, which comes into contact with 

 the pterygoids. The latter bones separate widely the 

 small palatines. Behind the latter bones are the posterior 

 palatine foramina. The pterygoids are greatly developt 

 anteriorly and laterally. They are in contact on the mid- 

 line in front, but for more than half their length the 

 basisphenoid bone comes between them. The pterygoids 

 are abbreviated behind, so that they permit the quadrates 

 to join the basisphenoid. This arrangement is character- 

 istic of the Pleurodires and distinguishes them from all 

 other turtles. The quadrate is notcht behind for the 

 passage of the stapes. 



The lower jaw is slender and the coronoid processes 

 are low. The articular furnishes a ball for articulation 

 with the quadrate. There is present what Baur called 

 a presplenial, but which is here regarded as the true 

 splenial, a bone absent from most turtles. 



The hyoid apparatus is greatly developt, but need not 

 here be described. 



The scapula is a strong bone. The procoracoid 

 *ith lower jaw; p rQ cess makes less than a right angle with the body of 



vom, vomer. , . -p. . . . . . . 



the bone, between the two portions, in the angle, is a 

 sharp crest. The glenoid fossa is at the end of a long neck. The coracoid is 

 relatively short, much bent in a horizontal plane, and expanded at the free end. 

 The humerus resembles that of the emyds in most respects. The ulnar and 

 radial tuberosities are somewhat larger, the ulnar ascending slightly above the head, 

 the radial descending lower than in the emyds. The ulna has no suggestion of an 



FlG. 6. Hydromedusa. Skull 

 from below. Xj. 



hoc, basioccipita!; bsp, basisphenoid; 

 exoc, exoccipital; mx, maxilla; pa, 

 parietal; pal, palatine; paoc, paroc- 

 cipital; pmx, premaxilla; pt, ptery- 

 goid; qu, quadrate; qu. art, articu- 



