THALASSEMYDID&. rt 51 
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 
Mena monic, 
, 
oe 
Fics. 185-188.—Catapleura repanda. Portions of shell of type. x 3. 
185. Nuchal, and first and second peripherals. nw. p, nuchal bone; per. 1, per. 
2, first and second peripherals. 
186. 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. wieland:), 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. 
