CHEL0N1A. 55 



Trionyx marginatus. Owen. Tab. XIX+. 



A more obvious character than that pointed out at the peripheral border of the 

 costal plate in Trionyx incrassatus, and serving better and more readily to determine 

 such elements of the carapace, is the ridge with minute parallel striae, which extends 

 along the upper surface, close to the anterior and posterior borders of the costal plates 

 in the species of Trionyx which 1 have on that account distinguished by the specific 

 name of marginatus. 



Mr. Erxleben has well given this character in the reduced view of the carapace 

 (T. XIX+). 



The border-pattern gradually becomes narrower and fades away before it reaches 

 the outer end of the costal plates ; it is also wanting on the anterior border of the 

 first costal plate (ph), and on the posterior border of the last (pis). 



The outer ends of the costal plates, which constitute the greater portion of the 

 periphery of the carapace, are at first slightly bevelled off, and then vertically truncate ; 

 the sloping or bevelled part having the fine fibrous surface, which I have compared 

 to coarse linen cloth. The vertical part of the border is slightly excavated in the 

 fifth and sixth costal plates, but not so deeply as in the Tri. circumsulcatus, nor is the 

 margin so thick in proportion to the length of the plate. 



The neural plates are relatively smaller, in comparison to the costal plates, than in 

 any of the foregoing species, but they agree in number ; the eighth being suppressed, 

 and the seventh reduced, in the same proportion as in Tri. Henrici and Tri. Barbara, by 

 the median union of part of the seventh pair and of the eighth pair of costal plates. The 

 fifth neural plate presents a simple oblong quadrilateral figure ; the four neural plates in 

 advance are six-sided, the two additional and shortest sides being formed by the 

 truncation of the posterior angles ; the sixth and seventh plates, on the contrary, have 

 their anterior angles cut off. This modification in the form of the neural plates, and 

 in their mode of juncture with the costal plates, relates to the opposite curvatures or 

 inclinations of the costal plates, in the direction of the axis of the carapace : the 

 anterior ones bending forwards, the posterior ones backwards, in addition to the curve 

 common to all but the last pair of plates, transversely to the axis of the carapace, 

 with the concavity downwards or towards the thoracic-abdominal chamber. The 

 anterior internal angle of the second, third, and fourth costal plates is cut off ; the 

 posterior internal angle of the sixth, and both internal angles of the fifth pair of 

 plates {ph). 



In these modifications of the form of the neural and costal plates, the Tri. 

 marginatus agrees with the Tri. Henrici and Tri. Barbara, and differs from the Tri. 

 incrassatus, in which all the neural plates but the seventh are six-sided, with the 

 posterior internal angles truncated, Each of the costal plates, therefore, of the fifth 



