250 Pibld Columbian Museum — Geology, Vol. III. 



The shell is in general structure broad and low, flat and emargi- 

 nate at the interior end, but steep posteriorly. The carapace and 

 plastron are firmly united by a wide bridge which extends more than 

 half the entire length of the shell. The whole of the free surface is 

 marked by pittings which vary in depth, structure, and arrangement 

 in different parts of the shell. The horny shields are outlined by 

 narrow sulci which are plainly marked in the vertebro-costal region, 

 and in the plastron, though less conspicuous between the marginals. 

 The sutures joining the bony plates are everywhere closed, so that 

 in some places they cannot be traced by the aid of a lens. The centra 

 of the dorsal vertebrae have been detached and lost from the speci- 

 men entirely. The neural plates bear on their inferior surfaces an 

 interrupted median ridge indicating the attachment of the neural 

 spines. The proximal ends of the pleural plates bear stout tuber- 

 cular processes for vertebral attachment. Plate LXXVII. 



The carapace is relatively thin throughout. The anterior end 

 is unusually flat, a character which has evidently been accentuated 

 by compressure in fossilization. The nuchal border is rounded and 

 indented by a wide concavity. Elsewhere the free margin is drawn 

 to a sharp angle. The posterior end is markedly convex in the pygal 

 region. The margin is slightly recurved over the femora, but uni- 

 formly convex and overhanging at the median line. Plate LXXVII. 



The neural plates are irregularly hexagonal, but vary in size and 

 in outline. A certain amount of asymmetry is also noticeable in 

 them. The first is broadly coffin-shaped, with the large end directed 

 backward. It articulates laterally with the first pair of pleurals, 

 postero-laterally with v the second pair, and posteriorly by a concave 

 line with the second pleural. The second is sub-ovate in outline 

 and notably smaller than the first. Its pleural articulation is con- 

 fined to the second pair. The third neural is a trifle smaller than 

 the first and has an outline similar but reversed in position. Thus 

 it articulates antero -laterally with the second pair of pleurals and 

 laterally with the third pair. The entire fourth and part of the fifth 

 are missing from this specimen. They were evidently narrower than 

 the third but similar in form. The sixth is notably asymmetrical. 

 The anterior end and right side have the usual form, but the left 

 side is elongate so as to articulate with the whole mesial end of pleural 

 vi and postero-laterally by a short line with pleural vn. The 

 seventh neural is thus excluded from its normal articulation on the 

 left with pleural vn, and is correspondingly asymmetrical. It is 

 also much smaller. The eighth neural is much shortened antero- 



