CuAP. 11. THE CIIELONIOIDiE. 327 



gidiflre, the plastron narrows continually backward from where it is first joined to 

 the carapace ; it is finnly wedged in between the curved edges of the carapace, 

 and cf)nsists of a thick, stiff sheet of unossified fibrous corium, and strengthened 

 onl}^ by a ring of bones of the true skeleton. In Chclonioidro, however, as the 

 plastron spreads out broader at the middle, as it meets the carapace at a sharp 

 angle, as it is connected with it by fiexiblo coriuui, and as it is somewhat flexilde 

 within itself, it also needs a different structure. It, is made up partl}^ of unossified 

 corium, and partly of plates composed of true bone anil of bony derm. These plates 

 form by far the larger part of the whole, and sometimes nearly the whole plastron. 

 The tAvo kinds of l)one are so united as to be hardly distinguishable ; we shall 

 therefore speak of the plates without reference to their composition. There are nine 

 of them, four pairs and one odd one. The first pair is situated between the front 

 limbs; they meet in front and spread apart backward, and overlap the outside of 

 the front edges of the next pair, which are here turned forward ; at their ends, 

 where they lueet, the}'^ are broad and strong, but grow slender backward. Joined 

 to the hind edges of this pair, and reaching back somewhat between the inner 

 edges of the second pair, is the odd plate ; it is interposed agauist the front pair 

 at their ujiion, and prevents the formation of a hinge in that end of the plastron. 

 These three plates, thus imited, make a broad, fina support for the shoulder appa- 

 ratus. The second and third pairs reach across from one edge of the carapace 

 to the other. These tvv^o pairs are sutured to one another, and together they 

 make up much the largest part of the plastron ; their outer edges are coimected 

 Avith the marginal rim by unossified corium, and their inner edges with one 

 another in the same way, but they approach the marginal riui and one another 

 by spine-like processes reaching out from near the fore end of the second and 

 tlu' hind end of the third pair. The fourth pair underlie the pelvis and meet 

 behind it ; they are long and slender, extending more backward than in\vard, and 

 are joined, before, to the third i>air. 



lu this faiuily, then, the dermal shieM is much more extended and more bony 

 than in the Sphargidida; ; the wings and }»addles are more covered hy the shield 

 and less free, and the body is ui.tn-e flattened upon the sides and below. These 

 characters seem to indicate that the animal is less capable of powerfid and long- 

 coutiiuied fiight. 



The shield is everywhere covered with ejiidermal scales. These scales are 

 largest upon the carapace, Avhere there is one median row ah^ng the A'ertebral 

 column, and one on each side above the costal plates, besides the row whicli 

 protects the marginal rim ; the foremost of tliese is an fxld, short, but very broad 

 scale ; the hindmost, on the contrary, form one pair. Upon the plastron there is a 

 double row of larger scales in the middle, and a row of smaller ones on each side 



