1 1 14 



CLASS REPTIL1A. 



Closely allied to the preceding is the genus Thalassochelys, as 

 represented by the Loggerhead (fig. 1006). The skull is, however, 

 relatively smaller in proportion to the shell, which is distinguished 

 by the presence of five costal shields • the carapace is heart-shaped ; 

 the xiphiplastrals are narrow, and join only at their extremities ; 

 while the terminal phalangeals are flattened and have but one or 

 two claws. The shell is distinguished from that of Chelone by its 

 more complete ossification, and its T-shaped entoplastral ; while 



Fig. 1019. — Oral aspect of the left half of the mandible of Lytoloma crassicostatum (a), and 

 Chelone mydas (b). Reduced. The former from the Lower Eocene of Belgium, the latter re- 

 cent, e, Postarticular process ; o, Splenial ; n, Geniohyoid groove ; g, k, Insertion of temporal 

 muscle; A, Coronoid process. 



the humerus is more constricted, and has its head placed more 

 obliquely. The position of the posterior nares and the form of the 

 palatal walls and mandibular symphysis closely resemble the same 

 parts in the young of Lytoloma. In very old specimens the vacui- 

 ties between the costals and marginals completely close up. This 

 genus probably occurs in the London Clay, 1 and also in the Middle 

 Eocene of Bracklesham. 



1 As indicated by a mandibular symphysis in the British Museum. 



