OSTEOLOGY. 



may be supposed to represent the costal plates is the irregular border of the some- 

 what expanded ribs. Of the other elements entering into the carapace of an emyd 

 or a pleurodirid, the leatherback possesses only the nuchal bone. 



But the leatherback has a carapace peculiar to itself. This is composed of a 

 layer of thin, polygonal bones which are buried in the thick skin of the animal. Of 

 these bones there are 7 rows of larger ones, that appear in the living animal as so 

 many sharp dorsal keels. One of these rows is along the midline; three run along 

 each side. In front, this layer of mosaic-like bones overlies the nuchal. Smaller 

 bones fill up the spaces between the rows. 



On the inferior side of the turtle there are 5 rows of similar bones, but the spaces 

 between the rows are not so completely filled as on the upper side. Beneath the 

 skin which supports these rows of bones there is a ring of elongated bones which 

 represent the plastron of more normal turtles. These represent the epiplastra, 

 the hyoplastra, the hypoplastra, and the xiphiplastra. The entoplastron is missing. 

 All these bones are slender and thin, and they surround a vast fontanel. 



FlG. 7. Dermochelys coriacea. Greatly reduced. 



The cervical vertebrae differ in no important respect from those of Caretta, and 

 the neck is equally short. The dorsals are ten in number and immovably joined to 

 those in front and behind by rough articular ends. The neural arches are moved 

 forward, so that each articulates about equally with its own centrum and that in 

 advance. They are somewhat expanded above, but do not come into contact with 

 plates from the ribs. In fact, these plates are extremely vestigial. There are two 

 sacral vertebrae, whose ribs articulate with the ilia; and there are about twenty 

 caudals. 



The skull at first glance presents many resemblances to that of members of the 

 Cheloniidae. The temporal roof extends backward as far as the occipital condyle. 

 The postfrontal and the jugal are large, and the squamosal joins the parietal. 

 The supraoccipital spine is short. The prefrontals extend backward to beyond the 

 orbits. The external nares look forward and strongly upward. The maxillae are 

 not strongly developt; and they have hardly any triturating surfaces. The choanae 

 are placed far forward and open directly into the roof of the mouth. A splint-like 

 vomer separates them, and extends backward to the pterygoids. The palatines are 

 broad, and they reach forward nearly to the vomer, sometimes coming into contact 

 with it. Thus, they bound the choanae outwardly. The pterygoids join on the mid- 

 line for a short distance only in front; otherwise, they are widely separated by the 



