58 FOSSIL REPTILIA OF THE LONDON CLAY. 



Trionyx planus. Owen. Tab. XIX C. 



This species, like the Tri. rivosus, is represented by the posterior part only of the 

 carapace, but the distinguishing characters are so well marked in it as to leave no 

 doubt respecting the difference of the species from that of any of the above-defined 

 Trionyces. The specimen consists of the last four pairs of costal plates, which are flat, 

 with a coarse reticulate pattern on their upper surface, worn away towards the median 

 end of the plates into a fossulate pattern, or detached pits ; the reticulate sculpturing 

 extends to the peripheral border of the costal plates, which is almost vertically cut 

 down, and is scarcely at all produced where the attached rib projects : there is no 

 marginal pattern along the anterior or posterior borders of the costal plate. The ribs 

 are more neatry defined from the superincumbent costal plates than in any of the 

 foregoing species, except, perhaps, the Tri. rivosus. The Tri. planus differs from them 

 all in the complete obliteration of both the seventh and eighth neural plates, and by a 

 partial obliteration of the sixth neural plate. This arises from a similar encroachment 

 of ossification from the postero-internal borders of the sixth costal plates, upon the 

 dermal cartilaginous matrix of the sixth neural plate, to that which happens in resj)ect 

 of the seventh neural and costal plates in the other Trionyces ; whilst the whole of the 

 seventh neural plate is superseded, as well as the eighth, and by the same encroach- 

 ment of the corresponding pairs of costal plates. 



These modifications and varieties of the osseous parts of the carapace are very 

 significative of the essentially dermal nature of those parts, and show the small value 

 and deceptive tendenc} r of that developmental character on which Cuvier and Rathke 

 have relied in pronouncing the neural plates to be developed spinous processes of 

 vertebras, and the costal plates to be expanded ribs. The connation of the seventh 

 and eighth neural plates with the corresponding costal plates does not destroy their 

 essential nature and existence, though it seems to make them part of the costal plates, 

 any more than that connation with the neural arch in other Chelonia which seems to 

 make them spinous processes. 



Another distinctive character in the Tri. planus, as compared with the foregoing 

 Eocene species, is the very close union, almost amounting to confluence, between the 

 seventh and eighth costal plates of the same side, the original suture between which 

 has been almost obliterated at their inferior surface. 



In this character the Tri. planus resembles the Tri. ferox, Schweigger {Gymnopus 

 spiniferus, Dum. and Bibr.), and Tri. muticus, Lesueur, but it differs from both by the 

 flatness of its carapace, and the absence of any keel-like elevations upon its outer 

 surface. 



The middle of the posterior border of the carapace is slightly concave. 



The specimen here described and figured was obtained by the Marchioness of 



