68 A FOSSIL CHELONIAN REPTILE, FROM THE MIDDLE PTOBECKS. 



Pleurosternon belongs to the second group, and is distin- 

 guished from the rest of the family by having an additional 

 number of inferior rib elements, comprising the under-shell or 

 plastron, also by the union of the carapace and plastron by 

 marginal plates. The carapace is less oval than that of the 

 marine Chelonians, approximating to the Order Testudinidso or 

 Land Tortoises in this respect, but differing in its slighter 

 convexity, and being nearly flat. The individual under 

 consideration is seventeen inches long, and fifteen inches broad. 



There are five marginal plates, mS, m ( J, wlO. mil, m!2, and 

 the pygal plate on the right side, portions of two mS and ml 2, 

 as well as the impressions of two more on the left side. Four 

 of the eight costal plates, pi. 1, pi. 2, pi. 3, and pi. 4, are 

 scarcely mutilated. The fortunate removal of the marginal 

 plates on the anterior end exhibits the beautiful symmetry of 

 the carapace. The first neural plate, which is entire in ordinary 

 Chelonians, is divided into two, by a transverse suture, the 

 second neural plate, 2, is pentagonal, the third, fourth, and 

 fifth, s3, 4, s5, are attached to the costal plates, pi. 2, pi. 3, and 

 pi. 4 ; the fifth and tenth neural plates, so, slO, are fragment- 

 ary ; the remainder, the sixth, seventh, eighth and ninth, 6, s7, 

 s8 aud 9, are wholly lost. The pygal plate is entire, and articu- 

 lated to the tenth neural plate. The surface of the costal 

 plates which are uninjured, shew the markings of the boundary 

 lines of the epidermoid scutes, which are coincident neither 

 with the costal nor vertebral plates ; for instance the first costal 

 plate, pi. 1 , is impressed by the boundary lines of the second 

 marginal scute, of the first and second vertebral scutes, and of the 

 first costal scute, uniting with the nuchal plate h (which is absent), 

 and the first and second marginal plates, ml and m2. The removal 

 of the osseous covering of the carapace, by which nearly the whole 

 of the left side of the endoskeleton is exposed, affords a favour- 

 able view of the vertebral column and the flattened series of 

 ribs. The vertebrse are flattened and attenuated, and would be 

 unsuited for the support of so massive a body, if they had not 



