CiiAP. II. THE TRIONYCHID.E. 329 



SECTION IV. 



THE FAMILY OF TRIONYCHID^. 



This flimily was first distinguished by J. E. Gray, and afterwards adopted by 

 Bell, Fitzinger, Wiegman, Canino, and Dumeril and Bibron, while Wagler unites it 

 with the other fresh-water Turtles.' 



The form of the Trionychidce resembles a flat orbicular disc, slightly elongated, 

 with a long, pointed head projecting upon a long, slender neck, and two pairs of 

 limbs, one before and the other behind, with broad, webbed feet moving hori- 

 zontally. 



The body is low, flattened, and spread out wide. The upper surface nowhere 

 arches high above the outer edge, either crosswise or lengthwise. The middle Une 

 above, along the dorsal vertebral column, or rather the cord of its slightly curved 

 arc, is very nearly parallel to the flat lower surface upon which the body rests. 

 From this middle hne the upper surflice descends slowly on either side toward the 

 outer edge, lowest about the shoulders in the arch of the third pair of rib.s, less 

 and less backward, until over the pelvic region the arch is very slight. As this 

 line is parallel to the base upon which the body rests, the outer edge of the 

 shield I'ises as the upper surface flattens, that is, from the shoulders backward. At 

 the shoulders it is but little above the flattened part of the lower .surface, so that 

 there the bulk of the body is above the plastron and within the arch of the cara- 

 pace, while at the hind end it is below the carapace and within the inverted arch 

 of the pla.stron. The opening in the shield for the protrusion of the limbs and 

 tail about the hind end is as high or higher than that about the front end 

 for the protrusion of the head and front limbs. The body is hhmtly curved 

 about the front end ; it is much broader across the shoulders than across the pel- 

 vi.s, and more pointed behind than before, but the projection of the marginal rim 

 beyond the body gives very different proportions to the carapace. This rim pro- 



* This fiimily corresponds exactly to the genus 'NViegniann changed the name to Chilota^ (Handb. 



Trionyx of Geoffroy, from which its present name is Zool. 1832.) Dum($ril and Bibron introduced a third 



derived. Gray writes the family name Trionicida", n.ime for this same family, calling it Potamides, 



(Ann. I'liil. 182."),) and Trionycida-, (Cat. Brit. Mus. (Erjict. gencr. 183.").) The name borrowed from the 



1844;) as also does Canino, (Saggio An. Vert. 1831.) genus Trionyx, having the priority over those of 



Bell writes it Trionichida;, (Zool. Jour. 1828.) Fit- Dumeril ami Bibron, and of 'Wicginann, must be 



zinger has it Trionyehoidea, (Neue Classif. 1820.) retained ; but it must be spelled TuioNYCiim.F. 



42 



