42 FOSSIL REPTILIA OF THE LONDON CLAY. 



surface of the neural and costal plates is so minutely fibrous or striated as to seem at 

 first sight almost smooth. The upper surface of the costal plate seems naturally to be 

 slightly concave in the direction of the axis of the carapace, but not so much as in 

 Chel. crassicostata, and the rib is much bent lengthwise. 



No. 9. Chelone crassicostata (T. XIII^). This instructive specimen is contained in a 

 subspherical nodule, 13 inches long by 12 inches broad, exposing a large proportion of 

 the outer surface of the carapace, with more than one half of the circle formed by the 

 marginal plates (w; — py). The carapace has been fractured, and the ribs of the left 

 side dislocated and pressed down below those of the right. The third {ph) to the eighth 

 {jjIs) costal plates inclusive are present on the left side; the fifth to the eighth on the right 

 side, and the neural plates from the fourth to the pygal plate (joy) inclusive. The fourth, 

 fifth, and sixth neural plates are hexagonal, with the anterolateral sides shortest, and 

 chiefly remarkable for their great breadth in proportion to their length. The seventh 

 and eighth are small, and more regularly hexagonal. The ninth is a broad sub- 

 crescentic plate, with the broad concave side backwards, and the space between this 

 and the pygal plate is filled up by an equally broad but pentagonal neural plate. The 

 length of the ninth and tenth neural plates, with the pygal plate inclusive, is 2 inches 

 9 lines. The pygal plate is subquadrangular and broadest behind, where it is slightly 

 emarginate. The length of the fourth to the eighth neural plate inclusive is 3 inches 

 8 lines. The upper surface of the bones of the carapace is almost smooth. That of 

 the costal plates is chiefly remarkable for its concavity transversely, or in the direction 

 of the axis of the carapace, which is to a greater degree than in the Chel. subcristata 

 or Chel. longiceps ; the lines of the sutural union of these plates with each other forming 

 so many ridges across the sides of the carapace. The degree of curvature or convexity 

 in the direction of the length of the costal plate is much greater than in the Chel. 

 planimentum. The length of the third costal plate is 3^ inches, its breadth at the outer 

 extremity, 1 inch 4 lines ; the breadth of the rib where it projects beyond it is 9 lines. 

 The margin of the plate attached to that rib is 1 inch 4 lines in length, and 8 inches in 

 breadth. The margin of the plates gradually increases in breadth towards the posterior 

 part of the carapace, the one joining the pygal plate being 1 inch 2 lines in breadth. The 

 general form of the carapace of the CIlcI. crassicostata is shown by the present specimen 

 to have been that of a full oval, with a gently festooned border, not pointed behind. 



No. 10. Chelone crassicostata (T. XIII5.) A still more remarkable example of 

 this species was kindly transmitted to me by the Rev. S. N. Bull, M.A., of 

 Harwich, of which a figure is given in T. XHI^. When it first came into my 

 hands it was an unpromising semioval nodale, 10 inches in length by 7 inches 

 in breadth, presenting on its convex surface portions of the posterior neural and 

 costal plates, with their external surface entire ; but no trace of plastron on the 

 flattened side. The degree of convexity formed by the costal plates equalled that of the 



