CUA?. III. 



GENERA OF CIIELYDROID.E. 



400 



lines, but so for exapfgerated as to appear one of the most aberrant representa- 

 tives of the whole family ; whilst it is so subdued in the most common species 

 as hardly to be perceptible. This being the case, I feel justified in saying, that, 

 whosoever does not see that all Naiades have the same form, is still as far 

 behind in Animal Morphology as the tyro in Geometry, who could not understand 

 that the circle may belong to a series of which the straight line would be an 

 extreme case, and again form another series with the ellipse, the parabola, and 

 the hyperbola ; with this fundamental difference only, that all these forms belong 

 to an unstable equilibrium in the organic world, whilst they have fixed relations 

 in the inorsranic. 



SECTION VI. 



THE GENERA OF CHELYDROID^. 



I know ouly three genera belonging to this family, and am not aware that 

 there exist others even remotely allied to them. These are the genus Cheltdr.\. 

 Sch/i'., the genus Platysternum, Gra^, and the genus Gypociielys, characterized in 

 this work for the first time. The genus Chelydra was characterized by Schweigger' 

 in 1812; Fleming^ called it Chelonura in 1822; Latreille^ called it Saurochelys in 

 1825; in the same year J. E. Gray'' gave it the name of Rapara; and in 1835, 

 Dumeril and Bibron,^ overlooking the many names already proposed by their 

 predecessors, insisted upon giving it another new one, Eraysaurus, which thej- spell 

 also Emysaura, and which has occasionally been further (quoted under the form 

 of Eraydosaura.'' The genus Platysternum was first characterized by J. E. Gray" 

 in 1831. Though I never had an opportunity myself of examining this last genus, 

 I have no doubt that it belongs to the family of Chelydroids ; and the descriptions 

 and figures given by Gray, and Dumeril and Bibron,^ furnish satisfactory evidence 

 of its true relations. This being the case, it is interesting to notice how widely 

 apart from one another the few living representatives of this family are found 

 upon the surface of our globe. Platysternum with one species, in China ; and Che- 

 lydra and Gypochely.s, each with one species, in North America. But this singular 

 geographical distribution acquires a special interest when it is further stated, that 

 the American genera Chelydra and Gypochel^s are only met with on the east- 



' In ilic work ([. a.. ]^. 394, note 5. 



^ In his I'liil. ZoiJl.. vol. 2, p. 270. 



* Families n.'itiiri'llos dii Ri'gii. An. 



* Aim. of PhU., 1825, vol. 10, p. 210. 



62 



» Erp. gen., vol. 2. p. 199, and 318. 



» Cat. Brit, Mus., 1844, p. 34. 



' Proc. Zool. Soc, London, 1831, p. lOG. 



» Erp. gen. PL IG, fig. 2. 



