250 FizLtp CoLuMBIAN Museum — Geo voey, Vot. 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 ofa lens. The centra 
of the dorsal vertebre 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 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 
v1 and postero-laterally by a short line with pleural vir. The 
seventh neural is thus excluded from its normal articulation on the 
left with pleural vi1, and is correspondingly asymmetrical. It is 
also much smaller. The eighth neural is much shortened antero- 
