OSTEOLOGY. 13 
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 Pelomedusidz 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In none of the Pleurodira is the neck withdrawn into the shell as it is in the other 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 pec uliar structures. ‘That of H ydrome- 
dusa 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 om above the whole upper surface of the ptery goids and the 
whole temporal fossee 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 premaxille 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 
Fic. 6.—Hydromedusa. Skull 
Pe ieee ean a presplenial, but which is here regarded as the true 
aes : 
nee _ splenial, a bone absent from most turtles. 
boc, basioccipital; bsp, basisphenoid; 
exoc, exoccipital; mx, maxilla; pa, The hyoid apparatus is greatly developt, but need not 
parietal; pal, palatine; paoc, paroc- here be described. 
Bee ae ae ee The scapula is a strong bone. The procoracoid 
lation of quadrate with lower jaw; process makes less than a right angle with the body of 
cei fs 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 
