30 CASE. [VoL. XIV. 
The xiphiplastron is an elongated bone articulating by an 
interlocking joint between the posterior processes of the hypo- 
plastron. It differs from all sea turtles in the peculiar bending 
of the bone near its middle; originally directed inwards and 
backwards, it changes its course abruptly, and is directed 
inwardly at almost a right angle to its original course. It 
articulated strongly with the xiphiplastron of the opposite side 
(Pl. IV and Pl. V, Fig. 9). 
There is nothing preserved of the ento- or epi-plastron. 
The general shape of the plastron was broadly ovate with the 
posterior end truncate. The fontanelle was diamond-shaped, 
and bridged at its anterior and posterior extremities by the 
lateral processes from the different plates. 
The plastron stands midway between that of Dermochelys 
and that of the Chelonitdae. In Dermochelys the union of the 
hyo- and hypo-plastra is by the overlapping of the extremities 
of the slender bones which have lost their radial processes. 
The xiphiplastron is straight, and articulates by overlapping 
with the hypoplastron. 
In Protosphargis Cap., the plastral bones are more robust 
than in Dermochelys. The marginal processes are retained to 
some extent, and the hyo- and hypo-plastra articulate by the 
interlocking of a few digital processes. The xiphiplastron is 
straight, and articulates with the hypoplastron by overlapping. 
The Cheloniidae have a broad sutural union between the two 
plates of the plastron. The marginal processes are confined to 
the distal ends of the bones, leaving the edges near the suture 
round and smooth. An approximation to this state can be 
noticed in Protostega, where the marginal processes near the 
union of the plates are shorter than those on the ends. The 
nature of the processes varies among the members of the Che/- 
onttdae. In Lepidochelys kempt Garm. they are numerous 
and irregular, standing on a base that leaves the main body of 
the plate in a curve, thus forming an oval or rounded fontanelle. 
In Chelonia there are generally only three processes on the 
hyoplastron, two of which project from the body of the bone at 
right angles, and meet across the squarish fontanelle, while the 
other passes obliquely forward toward the epiplastron. 
