Chap. II. 



THE CIIELYDROID^E. 



341 



Bibron, under the name of Elodites Pleurod&res. Wagler was the first to notice 

 the characteristic lateral moval)ility of the neck of these Turtles;^ but neither 

 he nor any of the earlier herpetologists availed themselves of this remarkable 

 anatomical peculiarity to separate the fresh-water Turtles into minor groups. 



SECTION VI 



FAMILY OF CHELYDROIDJE. 



The family distinguished by Swainson^ under the name of Chelidrid^e rests upon 

 an unnatural combination of the true ChelydroidtB and the Clielyoidae, as char- 

 acterized in the preceding section. But, while such an association of these Tur- 

 tles is contrary to the principles of classification discussed in the first part of this 

 work, it seems more in accordance with the practice generally followed in similar 

 cases to adopt the name proposed by Swainson than to frame another for the 

 family characterized in the following pages. This is the more feasible, as Swain- 

 son himself considered the genus Chelydra as the type of the family. All the 

 other natui-alists who have written upon the Reptiles unite the Chelydroidas with 

 the Emydoidie. 



The body of the Chelydroidos is high in front, and low behind ; the middle line 

 along the fixed part of the vertebral column descends from its front end backwards; 



Plcuroderes, in particular, seems to me to have a deep 

 significance. All the other Turtles, even the Chelo- 

 nii, as far as their neck is flexible, bend it in the pcr- 

 peniliciiiar plane of the longitudinal axis of their 

 body, ill the shape of an S, more or less arched. 

 The Pleuroderes, on the contrary, turn it sidewise, 

 and conceal it under the projecting edges of the cara- 

 pace and plastron, in the same manner as the Birds 

 hide their heail un<ler the wing. Thus this anatomi- 

 cal (•liMiact<'r excludes the Pleuroderes entindy from 

 the MMtiiral progressive series which begins with the 

 Sphargidiihc and ends with the Testudinina, and 

 stamps them as a distinct tyiie, bearing among Testu- 

 dinata a similar relation to the two sub-orders of 

 Chelonii and Amydic, characterized above, (p. 308.) 

 as the Marsupials hear to the placentalian Mammalia. 

 There is even this remarkable analogy between the 



representatives of these two classes, that, as among 

 the Marsupials and the higher Mammalia the families 

 correspond, to a great extent, to one another, so also 

 the families of the I'leuroderes recall the families of 

 the other Testudinata. The Emydoid form of 

 Owen's Chelone Benstedi, from the chalk of Eng- 

 land, its small size, and its early appearance in the 

 geological series, render the supposition quite plausi- 

 ble, that it may as well be a Cludonioid Pleurodere 

 as a genuine Chelonioid. At any rate, it has in no 

 way the form of a marine Turtle. 



* See "Wngler's Naturliches System der Amphib- 

 ien, p. 214 and 218. 



= SwAissox, (W..) Natural History and Classifi- 

 cation of Fishes, Amphibians, and Reptiles, London, 

 1839, vol. 2d, p. 116. The family name ought to be 

 spelled Chelydroidx, and not Chelidrida;. 



