THALASSEMYDID.S. 



border. The hinder border both of this bone and'of the first peripheral is oblique and overlapt 

 the first costal, as did the nuchal. The hinder half of the upper border of the second peripheral 

 is broken away and we can not tell how much of it was sutured to the costal. There is no pit 

 in the second for the anterior outer angle of the hyoplastron, and it is not probable that the 

 latter bone extended so far forward. 



The third peripheral is 60 mm. long on the thickened and obtuse free border. On the inner 

 face is a deep pit, directed backward, for the rib-end of the first costal. Below this, in the pro- 

 jecting edge is a notch, possibly for a digitation of the hyoplastron. Of the fourth peripheral 

 there is present only a portion. Its lower face is 26 mm. wide, and this makes an angle of ninety 

 degrees with the upper face, the angle where they meet being rounded off. The upper borders 

 of the third and the fourth peripherals are broken away. Cope figured what he regarded as the 

 sixth peripheral, and there is a fragment of a peripheral, which he has markt as the sixth; 

 but it is impossible now to to say that the bone present is the one figured. The bone has one 

 face, apparently the upper, 44 mm. wide. Another face, somewhat concave, is separated from 



FIGS. 185-188. Catapleura repanJ 



Portions of shell of type. Xj|. 



185. Nuchal, and first and second peripherals, nu.p, nuchal bone; per. l, per. 



2, first and second peripherals, 



1 86. Portions of the eighth and ninth peripherals. Free border toward the right. 



187. Section across ninth peripheral. 188. Third or fifth costal. 



this by a subacute free border. Cope has figured what he regarded as the eighth and part of the 

 ninth peripherals. In reality, there is only a small portion of the eighth and about two-thirds 

 of the ninth (fig. 186). Evidently these belong to the right side, since the intermarginal sulcus 

 crosses in front of the rib-pit. The latter is in the hinder half of each bone. The upper border 

 of'the upper face of both these bones is missing, but that part of the face of the ninth which 

 remains is 47 mm. wide and is slightly concave. The lower face, also slightly concave, is 46 

 mm. wide. The inner face has been about 25 mm. wide. Fig. 187 is a section of the ninth 

 along the intermarginal sulcus. The outer free border of the bone is acute and has projected 

 slightly at the end of the intermarginal sulcus, making the free border somewhat repand. 



On the free border and between the two bones here described is a long notch which, during 

 the life of the animal, inclosed a distinct ossicle. This reminds us of similarly disposed ossicles 

 described by Wieland as occurring in his Lytoloma angusta (L. wielanJi), and which he was 

 inclined to regard as a part of a disappearing primitive osteo-dermal armor of turtles. From the 

 manner of growth of the horny scutes one would rather expect to find such bones, if a part of 

 the ancient armor, at the ends of the intermarginal sulci. 



