ORDER CHELONIA. IIO3 



freshwater Purbeck and Wealden beds. They are characterised by 

 the more or less heart-shaped carapace, of which the costals are 

 often imperfectly ossified, and do not unite completely with the 

 marginals. The plastron has, moreover, a vacuity in the centre, 

 which persists for a long period or throughout life, but it had large 

 axillary and inguinal buttresses for connection with the carapace. 

 The humerus has a very imperfectly developed head and a slightly 

 curved shaft ; and the limbs were not modified into paddles. The 

 nuchal bone of the carapace has no costiform processes ; and the 

 temporal fossae of the skull were more or less completely roofed 

 over by bone. 



In their cordiform, and frequently imperfectly ossified carapace, 

 the Acichelyida agree with the modern Chelonidce ; and since the 

 peculiar form of the neurals of Tropidemys is another feature only 

 met with elsewhere in the latter family, there are strong grounds for 

 regarding the one family as the direct ancestor of the other. This 

 family also exhibits certain signs of affinity with the Pleurodiran 

 Plesiochelyidce ; while the simplicity of the humerus is a very general- 

 ised character. 



The genus TAa/assemys, which includes some very large forms, is 

 characterised by its long and flat neural bones, in which the anterior 

 lateral surfaces are much shorter than the posterior ones ; while the 

 vertebral shields are narrow. The carapace is well ossified, and of 

 considerable thickness. The type species occurs both in the Litho- 

 graphic Limestone and the Kimeridge Clay of England ; the unde- 

 scribed Chelonian from the latter deposit at Ely, to which the name 

 Enaliochelys has been applied, being apparently identical. As al- 

 ready mentioned, the Portlandian Stegochelys may likewise be the 

 same ; and the genus is represented in the Dorsetshire Purbeck. In 

 the typical genus Acichelys {Eury sternum? Achelonia, Palceomedusa, 

 or Aplax) the neural bones of the carapace are flat, with short and 

 often indistinct antero-lateral surfaces, and the costals are well ossi- 

 fied, the vertebral shields of the carapace being comparatively wide. 

 This genus seems to be confined to the Lithographic Limestone. 



In Pelobatochelys, of the Kimeridge Clay, we have a large form 

 with a very imperfectly ossified carapace, in which the costals are 

 extremely thin, and the neural bones are long and six-sided. The 

 antero-lateral surfaces of the neurals are much shorter than the 

 postero-lateral, and in the fifth neural the latter surfaces are deeply 

 excavated. The neurals, especially in the hinder part of the cara- 

 pace, are strongly ridged, having the form of the ridge-tiles of a 

 roof; and the hinder part of the whole carapace is itself roof-like. 

 The vertebral epidermal shields were very wide, and the borders of 



1 This name is the earliest, but is preoccupied. 



