432 OLIVEE p. HAY 



left side has the horny scute eroded from the surface. The hone 

 beneath each of the fifth costal scutes is small. When a frag- 

 ment was dug out and treated with acid gas was liberated. 



About fifteen of the marginal scutes are present. Of these 

 nearly all retain patches of bone which correspond to the pro- 

 jections along the border of the carapace. These bones are 

 partially exposed outwardly by the wearing away of the pro- 

 jections against objects during the movements of the animal. 

 It has not been convenient to determine the position of all these 

 scutes on the margin. One however, is the left eleventh ; another 

 apparently the right twelfth. One, probably the ninth left, 

 seems to have a strip of bone 25 mm. long, which formed the 

 edge of the carapace under that scute. At this point may be 

 mentioned the nuchal scute. At the middle of its front border 

 there is a fragment of bone which responds readily on the apph- 

 cation of acid. 



The scutes of the plastron are present and they bear on their 

 inner surfaces those patches of bone which the writer judged 

 from the marks on the mounted skeleton must have been pres- 

 ent. As these are better displayed on specimens described below, 

 nothing more will be said about them. 



Now must be described another set of bones, the meaning of 

 which is yet to be determined. These are small, thin, flat plates 

 which are Ukely to be indicated anywhere on the surface that 

 was covered by the horny scutes. Often the plates themselves 

 are present and, after the bone is moistened, may be picked out 

 of their resting places. In other cases thej^ appear to have fallen 

 out during maceration. Sometimes they have evidentl}^ become 

 coossified with the surrounding bone; sometimes there is present 

 only a scar which seems to show that long before the death of the 

 animal the plate had been absorbed. Occasionally it is difficult 

 to determine whether or not a depression in the bone represents 

 one of these plates. The latter are usually more or less nearly 

 circular or polygonal, but are sometimes irregular in form. A 

 full-sized illustration of the lower face of the right fifth and sixth 

 peripheral bones of no. 6596 of the American Museum of Natural 

 History is here presented (fig. 7). A little above and to the 



