1872.] 425 [Cope. 
bones of unknown reference. There are also some slender curved bones 
which probably pertain to the plastron. 
As the bones were exposed by excavations in the yellow cretaceous 
chalk, sketches of their positions and relations were made, which aid ma- 
terially in the restoration of the animal. The upper layer of bones were 
those of most irregular form, as cranial and limb bones. Mingled with 
these, but often beneath them, were the ribs, while underlying all, were 
the large flat pieces here described as dermal. Adhering to the inferior 
surface of these was a layer of thin oyster shells with parallel striate sur- 
face, perhaps Inocerami. The ribs presented their heads upwards; so 
that taking all points into consideration, there is little doubt that the 
reptile was entombed on its back. 
The texture of the bones is peculiar. There are nowhere to be seen 
medullary cavities, and the bone is spongy, but very finely so, the tubules 
at the largest being equal in diameter to an ordinary pin, and generally 
considerably smaller. They are arranged in concentric series. There is 
no thick dense layer of the bone as in other tortoises, but an extremely 
‘thin one, which is hard and sculptured on the surface with minute grooves 
or pits. The tissue of the bone is very fragile, and has a fracture like 
the mineral enclosing it. Many of the bones, especially those of the 
dermal skeleton, are extremely attenuated on the margins, being no 
thicker than writing paper. 
There are twelve marginal bones. They are all characterized by their 
laminar form. The thinnest are those furthest removed from the middle 
of the sides. They consist of a single lamina slightly thickened within 
the margin, producing a slight convexity of the lower side. The proxi- 
mal part of the bone is an extremely thin plate with radiating ossification, 
and consequently more or less serrate margin. It extends some distance 
over the extremity of the rib, whose apex is received into a half pit or 
acuminate groove with abrupt termination, about one-sixth the width of 
the bone from the margin. : 
In following the marginals to the middle of the side, the edge, as usual, 
increases in thickness. The lower side becomes more convex, and the 
upper slightly concave. The edge is acute, and a very open interior en- 
tering angle at the middle. The lateral extremities of the marginals are 
irregular, terminating ina double series of closely packed digitations, 
which terminate freely, and enter into no suture. The pit receiving the 
extremity of the rib approaches the margin, which now develops an in- 
ferior lamina of bone. This encloses the end of the rib, and thins out 
laterally in contact with the superior plate. At first the inner lamina is 
Short ; later it is almost as extensive as the outer part of the marginal 
plate, causing the double appearance when fractured. As the marginals 
thicken, a distinct inferior plane becomes distinguished, separated from 
the interior face by an obtuse angle. The upper face near the margin is 
more concave. In the thickest and inferior face is also somewhat concave, 
and the edge quite acute. The lateral extremities consist as before of 
packages of digitations which easily break out. 
