14 CRYPXODIRA. 



entoplastron, and, on each side, an opiplastron, a hyoplastron, u 

 hj-poplastron, and a xipliiplastron. The epiphistra may bo regarded 

 as the homologucs of the clavicles, the entoplastron of the inter- 

 clavicle, and the other element's as a modification of the Stegoce- 

 phalian and lihynchocephalian " abdominal ribs." The entoplastron 

 is absent in the Cinosternida?, even at birth. In the early stage of 

 life, the shape of the epiijlastra and entoplastron are highly sug- 

 gestive of tlicir homologucs in other llcptiles and 8tegocephalians, 

 and the paired abdominal bones are separated by wide fontanellcs. 

 This embryonic condition is retained until late in life in the Chely- 

 drida) and throughout in the Chclonida), whilst in all other recent 

 Cryptodira tlie 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, Emys, Ci/clemys, and 

 Cinostenium. 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, Emys, Cijdemys, PlaUjsternuin) they are narrowly 

 separated from the latter by ligament, 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 (Chelydridae) or be entirely free (Chclonidoe). The space 

 between the body of the plastron and the marginals is called the 

 bridge ; it is particularly short or absent in those Testudinidie in 

 which the plastron is movable, and long and narrow in those forms 

 (Clielydridai and Staurotypus) in which the plastron is particularly 

 small, the whole shield being cruciform. In such Testudinida3 as 

 liave the plastron suturally united with the carapace, the hyo- and 

 hypoplastron each send up a process, respectively termed the axillary 

 and inguinal buttress, whicli anchylose cither with the inner surface 

 of the marginals oi- with the costals : these buttresses are least deve- 

 loped in the land Tortoises and most in the Testudinidte frequenting 

 deep water {Batayur, Kachufja, Gallayur, and Ilardella), in which 

 genera they form very largo 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- 

 tudiiiidte except Cyclemys, Geoemyda, and Nicoria, in wbicli it is 

 usually totally absent. Tbe entoplastron is dagger-shaped in Chidone 

 mydas. Tbc 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 Emys, as 



