12 BRITISH FOSSIL REPTILES. 



there is a l)eautiful Chelonite (PL 16, figs. 1, 2) including the carapace, plastron, and 

 the cranium, which is bent do\\Ti upon the fore part of the plastron ; and which, 

 though mutilated, dis2)lays sufficient characters to establish its specific identity with 

 the skull of the Chelone brevicej^s just described. Both the carapace and plastron 

 present the same finely rugous surface externally as the cranium ; in which character 

 we may perceive a slight indication of affinity with the genus Trionyx, 



The carapace (PI. 16, fig. 1) is long, narrow, ovate, widest at its anterior half, 

 and tapering towards a point posteriorly ; it is not regularly convex, but slopes away, 

 like the roof of a house, from the median line, resembling, in this respect, and its 

 general depression, the carapace of the turtle Chelone mydas. There are preserved the 

 nuchal plate (fig. 1, e/<*) with ten of the neural plates {u\ — ?^lo*), only the eleventh 

 and pygal plates being wanting. The eight pairs of costal plates {pl\ — -ph) are also 

 present, with sufficient of the narrower tooth-like extremities of the six anterior pairs 

 of ribs, to determine the marine character of the fossil, which is indicated by its 

 general form.f 



The nuchal plate (fig. 6, ch) is of a transversely oblong form, with the anterior 

 margin gently concave. Its antero-posterior diameter, or length, is ten lines ; its 

 transverse diameter, or breadth, is two inches. The lateral margins are bounded by 

 two lines meeting at a slight angle ; to the anterior one, the first of the marginal 

 plates, «n, is attached; the posterior line bounds part of the vacant interspace 

 between the first costal plate (jy/l), and the anterior marginal plate. The presence 

 of this plate would prove for the genus Chelone as against Trionyx, were the characters 

 of the cranium, the impressions of the vertebral scutes, and the sternum wanting. 

 The nuclial plate in the Emydes is hexagonal, and nearly as long as it is broad. 



The Chelonite from the tertiary beds near Brussels, figured by Cuvier,:}: has the 

 nuchal plate of nearly the same form as the present specimen from Sheppey. 



The neural plates in the Chelone breviceps axQ as narrow as in the Chclones generaWy ; 

 and as in the Brussels Chelonite above cited. 



The first neural plate (.si, fig. 1) is four-sided; the rest, to the eighth («8), are 

 hexagons of a more regular figure than in the existing Chelones, and are articulated 

 to more equal shares of the contiguous alternate costal plates {ph — ph).* 



The first costal plate {ph) is directed more outwards, does not incline backwards, 

 as in recent Chelones, and its anterior angle is less truncated than in them. (See 

 fig. 1, p. 3.) 



The length of the second costal plate [ph) is one inch, nine lines ; more than half 

 of the narrow terminal extremity of the connate rib is preserved ; the propoitions of 



* These letters refer to the parts in the typical carapace fig. 1, p. 3, by reference to which the cor- 

 icspondiiig parts in the fossil will be readily under.stood. 



f In an Emijs with a carapace seven inches in length, the corresponding extremities of the ribs would 

 have been united together by the laterally-extended ossification. 



X Osi-emens Fossiles, torn, v, pt. 2, pi. xv, fig. IG. 



