OSTEOLOGY. 15 
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. 
a - 
Fic. 7.—Dermochelys cortacea. Greatly reduced. 
The cervical vertebra 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 pont 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 
Cheloniidz. 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 maxillz are 
not strongly developt; and they have hardly any triturating surfaces. The choanz 
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 choane outwardly. The pterygoids join on the mid- 
line for a short distance only in front; otherwise, they are widely separated by the 
