1 1 16 



CLASS REPTILIA. 



shell the carapace is either heart-shaped, or pointed at each end 

 (fig. i o 2 1 ) ; the vacuities are large and persistent ; the entoplastral 



is long and dagger - shaped ; 

 and the xiphiplastrals are 

 slender and separate (fig. 

 1009). The humerus is but 

 little constricted, with its head 

 nearly on the axis of the shaft ; 

 and the coracoid is longer and 

 more slender than in Thalas- 

 sochelys ; the Green Turtle (C. 

 my das) being more specialised 

 in these respects than the 

 Hawksbill (fig. 1018). The 

 earliest occurrence of the 

 genus appears to be in the 

 Cambridge Greensand and 

 Gault, where C. Jessoni has a 

 very massive mandible, some- 

 what resembling that of the 

 Hawksbill. This form may 

 possibly be identical with C. 

 Benstedi of the Chalk (fig. 

 102 1 ), which is only definitely 

 known by very young shells, 

 and has been made the type of the genus Cimoliochelys. In the 

 topmost Cretaceous of Maastricht the gigantic C. Hoffmanni ap- 

 pears to be allied to C. imbricata, but has a shorter and wider 

 palate and mandibular symphysis, a more deeply emarginate nuchal, 

 and the costal bones extremely short. From the latter very special- 

 ised character Dr Baur regards this species as entitled to generic 

 distinction, and has proposed the name of Allopleuron. An equally 

 large, and perhaps closely allied turtle occurs in the English Chalk. 

 In the higher Miocene of Bordeaux C. girondica appears to be a 

 form closely allied to existing types. 



Section 4. Trionychoidea. — The last section of the suborder 

 includes the mud-turtles or soft-tortoises, of the freshwaters of Asia, 

 Africa, and North America ; all of which are of aquatic and car- 

 nivorous habits. These forms, which may probably be regarded 

 as extremely specialised types, present the following distinctive 

 features. The shell is sculptured and devoid of epidermal shields ; 

 its entoplastral being in the form of a chevron, which divides the 

 epiplastral from the hyoplastral ; and the marginals, if present, 

 forming only an incomplete series at the posterior extremity of the 

 carapace, and having no connection with the ribs. The long cer- 



Fig. 1021. — Carapace of Chelone (?) Benstedi; from 

 the English Chalk. Reduced. (After Owen.) 



