38 FOSSIL REPTILIA OF THE LONDON CLAY. 



it is in Chel. longiceps ; in one specimen of which, now before us, Professor Owen found 

 that the articulation in question was to the anterior part of the second, instead of the 

 posterior part of the first, neural plate ; in other words, that the first neural plate was 

 the isolated one instead of the second. The remaining neural plates are hexagonal, 

 becoming almost regularly shorter to the eighth ; the lateral angles meeting the costal 

 sutures being nearly at the same distance from the anterior margin in each, and in no 

 one at all approaching a regular equilateral hexagon, as in many of the neural plates in 

 Chel. breviceps. The first three, and the anterior half of the fourth neural plates are 

 flat ; but on the posterior half of the fourth commences a low carina, which becomes 

 highest on the posterior half of the sixth (s6), and anterior half of the seventh (sj). 

 It thus differs from Chel. subcristata, in which there is a distinct, short, sharp, longi- 

 tudinal crest (si) on the fifth and seventh neural plates, " and a similar crest is developed 

 on the contiguous ends of the second and third neural plates." The ninth and tenth 

 neural plates are wanting in the only specimen I have seen of the Chel. subcarinata. 



The first costal plate is flat (ph), but the remaining ones, to the seventh inclusive, 

 are slightly hollowed along the middle, being raised towards the anterior and posterior 

 margins, where they are articulated to the contiguous ones. The whole surface of the 

 bones of the carapace is less smooth than in most other fossil species, and conspicuously 

 less so than in Chel. subcristata. 



In describing the forms of the vertebral scutes, (vi — V4), and of the costal ones as 

 depending upon them, it is necessary, in order to arrive at any satisfactory comparison 

 between these parts in different species, to bear in mind that a great change takes 

 place in their outline during the growth of the animal ; and that a vertebral scute, 

 which, in a younger individual, has the middle of its outer margin exceedingly extended, 

 so as to form a very acute angle, where the lateral margin of the costal scute joins it, 

 and thus rendering it twice as broad as it is long, may in more advanced age have that 

 angle very open, and having increased greatly in length, and scarcely at all in breadth 

 from angle to angle, the length becomes greater than the breadth. Allowing, however, 

 for this fact, there are doubtless considerable variations in this respect according to the 

 different species, which are permanent and well marked. The first vertebral scute 

 (vi) in the present species is quadrilateral, broader anteriorly ; the second and third 

 (v2, v3) hexagonal, with the outer margins slightly waved, somewhat broader in the 

 middle at the angles than at the anterior and posterior margins, the comparative 

 breadth at that part being rather greater than in the corresponding scutes of Chel. 

 subcristata, and much less so than in Chel. convexa, Chel. breviceps, or Chel. longiceps. The 

 fourth vertebral scute (vi) is also hexagonal, but the portion posterior to the lateral 

 angles is narrowed and produced backwards. The last of the series is fan-shaped. 

 The outline of the costal scutes follows of course that of the vertebral ones. 



The plastron, in the specimen from which this description is taken (PI. VIII, A, 

 fig. 2), is more perfect than in that of almost any other fossil Chelonian I have seen. It 



