100 



or carapace, and the development of independent superficial dermal bones. 

 The dermal bones consist of large plates lying above tbe ribs, which have no 

 sutural union with each other ; of some small vertebral shields on tbe dorsal 

 line ; and of thin, marginal bones, which have no sutural union with each other 

 or with the other bones. The vertebras preserved possess ball-and-socket 

 joints, and have flat neural arches, with widely-spreading articular processes. 

 The humeri are flat, and furnished with an enormous deltoid crest. The fore 

 limbs were very long, and formed flippers like those of the marine turtles 

 of the present seas. The bones of the head are very light and thin, and 

 mostly united by scmamosal or overlapping sutures. The mandible presents 

 the elements usual in the marine turtles, and has no angle. It exhibits a 

 deep pterygoid fossa, and is very light. The constitution of the bones is 

 rather dense, and there are no medullary cavities whatever. The superficial 

 layer is very thin and striate. The bones are all very fragile. 



The affinities of this genus appear to be largely to the Sphargididce. 

 This family is represented, in our present knowledge, by but one genus and 

 one species of the recent seas. It is one of the most generalized, or, in 

 special characters, the most aberrant, of the order of tortoises, and the dis- 

 covery of an extinct ally, even as far down in the series as the Cretaceous 

 period, is not surprising. 



The remains preserved belong to a single individual, and include many 

 portions of the cranium, five vertebrae more or less incomplete, the scapular 

 arches of both sides, with the coracoid bones; both humeri perfect, with nine 

 phalanges, ten ribs, one vertebral(?), and ten marginal bones; parts or wholes 

 of four large lateral (?) dermal bones, with five distinct 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 mate- 

 rially in the restoration of the animal. The upper layer of bones were those 

 of must 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 surface, perhaps Lwceram't. 

 The ribs presented their heads upward, so that, taking all points into con- 



