CHELONIA. 57 



entire carapace ; they would be transverse to the long diameter of the detached costal 

 plate ; they become more wavy as they recede from the neural plates. Of these only 

 the sixth (^6) has been preserved in the specimen described ; it differs in shape from 

 that in any of the foregoing species, in being broader in proportion to its length ; its 

 greatest breadth being, as in Tri. Henrici, Tri. Barbara, and Tri. marginatus, across 

 its anterior fourth part. The fifth neural plate, as in the species above cited, has been 

 an oblong quadrate one, the fourth plate has had its postero-internal angles cut off, 

 contrariwise to the sixth. The fifth costal plates have accordingly the same character 

 of truncation of both their internal angles, though less marked anteriorly. A portion 

 of the seventh and the entire eighth neural plates have been superseded, as in the 

 other fossil Trio?iyces, by the median growth and junction of the seventh and eighth 

 pairs of costal plates. 



In the forms and proportions of these plates the present species agrees best with 

 the Tri. Henrici and Tri. incrassatus ; the latter species differs from it by the breadth 

 and convexity of the sixth costal plates {ph). The smooth connate ribs (5, 6, 7), 

 shown on the under surface of the costal plates, T. XVIIl^, fig. 2, preserve a more 

 uniform diameter, and do not expand in the degree shown in the Tri. incrassatus, 

 T. XVII, 5, 6, 7; the rib (s) attached to the costal plate {pli) is straighter in Tri. ricosus 

 than in Tri. incrassatus. 



The projecting extremities of the ribs are beautifully preserved in the specimen of 

 2Vi. rivosus here described : their greater length, as compared with those attached to 

 the fifth, sixth, and seventh costal plates in Tri. Barbara (T. XVI^), depends upon 

 the nonage of the present specimen, which is figured in T. XVlII^i of the natural 

 size. 



The peripheral borders of the costal plates are bevelled off obliquely from above 

 downwards, and project a little where they join the end of the subjacent rib ; the 

 surface of this is finely and longitudinally striated. The reticulate sculpturing of the 

 carapace extends to the sloping peripheral border, as it does to the vertical thick 

 border of the carapace in Tri. incrassatus ; it is not separated from the border by a 

 marginal decussating fibrous surface, as in Tri. marginatus, Tri. Henrici, and Tri. 

 Barbara. 



The longitudinal ridges of the carapace, which form the chief distinctive character 

 of the Tri. rivosus, off"er an interesting though slight approach to the main feature of 

 the carapace of the Luth or coriaceous soft turtle {Spar (/is coriacea) ; but in this 

 existing species the longitudinal ridges or carinae are straighter and more elevated, 

 and the surface of the carapace is smooth at the interspaces. The less parallel and 

 wavy course of the ridges in the present extinct Trionyx give a sinuous course to the 

 intercepted spaces, like the furrows left by streams of water which have temporarily 

 coursed over a sandy surface, whence the name " rivosus" proposed for the species. 



8 



