24 FOSSIL REPTILIA OF THE LONDON CLAY. 



bon} r palate is longer than in Clielone breviceps, shorter than in Chel. longiceps. 

 The symphysis of the lower jaw (T. VII, fig. 3) is longer or deeper than in the Chelonc 

 breviceps, but is convex below from side to side, and not flattened as in the Clielone 

 planimentum. 



All the specimens of Clielone convexa, which I have been able to determine, are 

 from the London clay of Sheppey. 



Chelone subcristata. Owen. Tab. VIII. 



Proceedings of the Geological Society of London, December 1, 1841, p. 576. Report 

 on British Fossil Reptiles, Trans. British Association, 1841, p. 179. 



The fifth species of Clielone from Sheppey, distinguishable by the characters of its 

 carapace, approaches more nearly to the Clielone caouanna in the form of the vertebral 

 scutes (vi — va), which are narrower in proportion to their length, than in any of the 

 previously described species ; but the Clielone subcristata is more conspicuously 

 distinct by the form of the fifth and seventh neural plates (ss, sj), each of which supports 

 a short, sharp, longitudinal crest; a similar crest is developed from the contiguous ends 

 of the second and third neural plates (s2, S3) ; the middle and posterior part of the 

 nuchal plate {ch) is raised into a convexity, as in the Chel. longiceps ; but not into a crest. 



The keeled structure of the above-cited neural plates is more marked than in the 

 third and fifth neural plates of Clielone mydas, which are raised into a longitudinal ridge. 



The neural plates in the present carapace have the ordinary, narrow, elongated form 

 of those in the true Chelones. The nuchal plate (ch) has the middle of its hinder border 

 produced backwards, instead of being emarginate, as in the Chel. breviceps (T. II,fig. \,ch). 



The first neural plate in the Clielone subcristata (T. VIII, si) resembles that in the 

 Clielone convexa, but is narrower in proportion to its length ; the second (52) is also 

 quadrangular, as in Chel. convexa, but is narrower ; the third to the seventh likewise 

 differ from those in Chel. convexa only by being narrower ; but the eighth and ninth 

 neural plates are relatively smaller than in any of the before-described fossils, and 

 resemble those of existing Chelones. The expanded plate is more elevated, and is 

 bent down on each side, with the middle part forming an obtuse longitudinal ridge. 

 A part of the contiguous portion of the first (pll) and the second {ph) costal plates 

 are raised into a slight convex eminence on each side ; the surface of the remaining 

 pairs of ribs is flat in the axis of the body, but they are more convex transversely 

 to that axis, and in the direction of then* own length, than in the other Chelonites. 



The whole outer surface of the bones of the carapace is as smooth as in the Chel. 

 longiceps and Chel. convexa. 



Length of carapace from the first to the eighth neural plate inclusive : 



Ch. subcristata. Ch. breviceps. Ch. longiceps. Ch. convexa. 



Inches Lines. Inches Lines. " Inches. Lines. Inches. Lines. 



74 56 59 58 



