CHELONIA. 13 



the remaining costal plates correspond with those of the Chelone mydas, and Cliel. 

 caouanna. 



The last pair of costal plates [ph) articulates with the eighth, ninth, and tenth 

 neural plates, but does not overlap or supersede any of them. 



Not any of the costal plates articulate with those of the opposite side, so as to 

 inteiTupt the series of vertebral plates, as in the carapace of the Chelone caouanna 

 (fig. 1, p. 3), as in Mr. Crowe's Sheppey Chelonite, figured by Cuvier (torn. cit. pi. xv, 

 fig. 12) ; and as is shov^ai in the view of the concave surface of the Brussels species 

 (torn. cit. pi. XV, fig. 16). 



The ninth neural plate (fig. 1, sg) is the narrowest, as in the Chelones, and as in the 

 Brussels Chelonite, figured by Cuvier, in loc. cit. pi. xiii, fig. 8, instead of beino- 

 suddenly expanded, as in most Emydes. 



The tenth neural plate («io) expands to a breadth equal with its length ; the 

 eleventh and pygal plates, as already observed, are wanting in the fossil. 



The vertebral or median ends of the costal plates present a modification of form, 

 corresponding with that of the interspaces of the neural plates to which they are 

 articulated. Only the first pair {pl\) present that form which characterises all but 

 the last pair in the existing Chelones, and in the Brussels Chelonite ; viz., a straight 

 line with the posterior angle cut off; the rest being terminated by two nearly equal 

 oblique lines, meeting at an open angle, as shown in PI. 16, fig. \,pl'l — ph. 



This character would serve to distinguish the Chelone breviceps, if only a portion of 

 the carapace, including the vertebi-al extremity of a rib, were preserved. The free 

 extremities of the ribs are thicker in proportion to the costal plates, than in the Chelone 

 caouanna, or the Chel. mydas ; and more resemble, in this respect, those of the Chel. 

 imbricata, the species characterised by the size and beauty of the horny scutes, 

 commonly called " tortoise-shell." 



More or less complete impressions of the five horny vertebral scutes {v\ — vb), and 

 of four costal scutes on each side of the vertebral ones, show the forms and proportions 

 of these characteristic parts, and especially of the median series, notwithstanding they 

 were among the soluble and perishable elements of this ancient turtle of the Thames. 

 The hexagonal vertebral scutes are characterised by the near equality of their 

 sides, and the angle of about 100°, at which the two outer sides meet. 



The anterior border of the first vertebral scute, i^, has crossed and impressed the 

 nuchal plate, ch, near its anterior border ; this scute has covered the rest of the nuchal 

 plate, and more than half of the first neural plate. The second vertebral scute, v-, 

 includes the rest of the first neural plate, the whole of the second, and almost the whole 

 of the third neural plate. The third vertebral scute, ^;^ includes the hind border of the 

 third neural plate, with the whole of the fourth and fifth neural plates. The fourth 

 vertebral scute includes the sixth and seventh, and very nearly the whole of the eighth 

 neural plates, and the outer angles of this scute terminate over the suture between the 

 sixth and seventh costal plates. 



