14 CRYPTODIRA. 
entoplastron, and, on each side, an epiplastron, a hyoplastron, a 
hypoplastron, and a xiphiplastron. The epiplastra may be regarded 
as the homologues of the clavicles, the entoplastron of the inter- 
clavicle, and the other elements as a modification of the Stegoce- 
phalian and Rhynchocephalian “ abdominal ribs.” The entoplastron 
is absent in the Cinosternide, even at birth. In the early stage of 
life, the shape of the epiplastra and entoplastron are highly sug- 
gestive of their homologues in other Reptiles and Stegocephalians, 
and the paired abdominal bones are separated by wide fontanelles. 
This embryonic condition is retained until late in life in the Chely- 
dride and throughout in the Chelonide, whilst in all other recent 
Cryptodira the plastron forms in the adult a solid shield, which may 
be divided into two or three portions by the presence of one or two 
transverse ligamentous hinges, as in Cistudo, Hmys, Cyclemys, and 
Cinosternum. It is well to state, for the fact has often been over- 
looked, or even denied, that this hinge is developed with age, the 
more distinct the older the specimen; it is absent, or very indistinct, 
in quite young specimens. In the adult of most genera the hyo- 
and hypoplastra are united with the marginal plates by suture; in 
a few (Cistudo, Hmys, Cyclemys, Platysternum) they are narrowly 
separated from the latter by hgament, the plastron being movable 
upon the carapace ; or the outer border of these bones form digitate 
dentations, which may either articulate by gomphosis with the 
marginals (Chelydridz) or be entirely free (Chelonidee), The space 
between the body of the plastron and the marginals is called the 
bridge ; it is particularly short or absent in those Testudinide in 
which the plastron is movable, and long and narrow in those forms 
(Chelydride and Stawrotypus) in which the plastron is particularly 
small, the whole shield being cruciform. In such Testudinide as 
have the plastron suturally united with the carapace, the hyo- and 
hypoplastron each send up a process, respectively termed the axillary 
and inguinal buttress, which anchylose either with the inner surface 
of the marginals or with the costals; these buttresses are least deve- 
loped in the land Tortoises and most in the Testudinide frequenting 
deep water (Butagur, Kachuga, Callagur, and Hardella), in which 
genera they form very large septa nearly reaching the vertebral 
region and forming two lateral chambers occupied by the lungs, 
which are particularly developed in correlation with the diving 
powers. 
In the embryonic stage the entoplastron or interclavicle is con- 
stantly longer than broad, rhomboidal or T-shaped, terminating 
posteriorly in a sharp point which persists, as a more or less elongate 
spine, on the inner surface of the plastron, in the adult of all Tes- 
tudinide except Cyclemys, Geoemyda, and Nicorta, in which it is 
usually totally absent. ‘he entoplastron is dagger-shaped in Chelone 
mydas. The shape and connections of the plastral bones in newly- 
hatched Cryptodira vary considerably according to the genera. 
The hyoplastra are widely separated from the hypoplastra in the 
land Tortoises, and in contact in the aquatic, broadly in Hmys, as 
