CHELONIA. 59 



Hastings from the Eocene sand of Hordwell Cliff, and forms part of her ladyship's rich 

 and instructive collection. 



With the above portions of carapace, and apparently belonging to the same species 

 of Trionyx, were found the two osseous plates, naturally and suturally united together, 

 which are figured in PI. 33, fig. 6, hs, ps ; they present a similar coarse reticulate 

 pattern on their external surface, with the same tendency to a concentric arrangement 

 of the raised parts towards the periphery of the plate ; their inner surface is smooth, 

 slightly undulating, but upon the whole a little concave, and without any indication of 

 adherent ribs. I regard them therefore as parts of the plastron, and they agree best 

 with the hyosternal and hyposternal elements of the right side ; yet differ in having 

 no tooth-like processes extending from the inner border, which is convex instead of 

 being concave, where the two elements join each other. 



At the inner and anterior angle of the hyosternal there is, however, the fractured 

 base of what was probably a tooth-like process ; and there is similar evidence of such 

 processes having extended from the posterior angle of the hyposternal, close to what I 

 take to have been part of the notch for the xiphisternal. 



These fragments at least show that the Tri. planus, or whatever species from 

 Hordwell they belonged to, must have had a very different form of plastron from that 

 of the Tri. incrassatus of the Isle of Wight, of which the conjoined hyosternal and 

 hyposternal bones are figured in PI. 28, figs. 3 and 3', and from that plastron of 

 which the hyposternal piece, from Bracklesham, is figured in PI. 33, fig. 7. 



Trionyx circumsulcatus. Owen. Plate 31, figs. 1, 2, and 3. 



It may seem to have been hazarding too much to found a species on a single 

 character when manifested by a single fragment of a carapace, which is all that at 

 present represents such species ; yet the character in question is so strongly marked, 

 and so different from that of the same part of the carapace of any other fossil or recent 

 species of Tnotn/.r, that there appears to be no other alternative than to regard it as 

 specific. The character in question is the groove or canal which is excavated in the 

 thick vertical margin of the expanded free extremity of the fourth costal plate of the 

 left side, figured in PI. 31, figs. 1, 2, and 3. The vermicular sculpturing of the 

 external surface of this plate, and its proportions and connexions with the connate 

 rib, prove it to belong to the carapace of a Trionyx. 



Previously to receiving this specimen from Lady Hastings, my attention had been 

 drawn to the different modes in which the extremities of the costal plates of the 

 different species of Trionyx were modified, in order to form the border of the carapace ; 

 sometimes obliquely bevelled down to an edge, as in the Tri. Barbarte and the 

 fragment of the Trionyx pustulatus, from Sheppy, figured in PI. 31, 7 — 10; some- 



