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teristic of Testudinata. The presence of dermal dorsal bones is of the same 

 kind. Tlie genus Protostega then belongs near the Sphargididce in the sub- 

 order Athecee, and is in some points to be approximated to the Cheloniidce. 



Protostega gigas, Cope. 



There are twelve marginal bones. They are all characterized by their 

 laminar form. The thinnest are those farthest 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 proximal part 

 of the bone is an extremely thin plate, with radiating ossification, and conse- 

 quently 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, with a very open interior entering angle 

 at the middle. The lateral extremities of the marginals are irregular, termi- 

 nating in a double series of closely-packed digitations, which terminate freely, 

 and enter into no suture. The pit receiving the extremity of the rib ap- 

 proaches the margin, which now develops an inferior lamina of bone. This 

 incloses the end of the rib, and thins out laterally in contact with the superior 

 plate. In some, the inner lamina is short ; in others, it is almost as extensive as 

 the outer part of the marginal plate, causing the double appearance when frac- 

 tured. As the marginals thicken, a distinct inferior plane becomes distin- 

 guished, separated from the interior face by an obtuse angle. The upper face 

 near the margin is more concave. In the thickest, the 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. 



A single nearly bilateral bone of this series appears to be either nuchal 

 or caudal; but, as it has no sutural connection with any other, it is not easy to 

 determine which it is. Its marginal length is much less than its transverse 

 extent, which consists chiefly of a flat lamina. The marginal part is a little 

 thickened, and bilaterally concave below, and correspondingly convex above. 

 The margin is thin and acute. A few grooves radiate at a distance from the 

 middle toward the margin. The lack of concave excavation of the margin 

 would incline the balanct; in iiivur of the view thai this hone is \\\v caiKlal. 



t) 



