Chap. II. THE SPIIARGIDIDiE. 321 



theoretical point of view, it is of the utmost importance to know that an iso- 

 lated genus may constitute a distinct family, because such a fact shows how futile 

 and artificial the efforts of those naturalists must be, who aim at establishing the 

 utmost equality between groups of the same kind. Here we have a natural fam- 

 ily, not only with a single genus, hut perhaps with a single species, or, at the 

 utmost, numbering two or three species, while there are other families, in which the 

 genera may be counted by tens, and the species by hundreds. 



The form of the Sphargidida3 may be compared to a flattened cone with angu- 

 lar sides, to which are appended in front a large head with a pair of larger 

 naked paddles, and behind, a smaller jiair of very broad rudder-s. 



The body is broadest about the arch of the second pair of ribs, where the 

 carapace and plastron first unite, and narrows gradually from thence backwards 

 to near the arch of the seventh pair of ribs, where the union of the carapace 

 and plastron ends. The portion of the vertebral column which is fixed descends 

 gently from the neck to the sacrum. Thus, that part of the body which is 

 entirely encircled by the shield forms a truncated cone with its base turned 

 forward. This cone is the more s;^Tumetrical, because the body is deep below 

 the plane of its outer edge and not so extensively flattened as in most Turtles, 

 but tapering downward, so that the median horizontal flat surface of the plas- 

 tron is quite small. The shield fits close to the body above and below, and 

 assumes the same conical form. The carapace, after passing over the thoracic 

 and abdominal regions and separating from the plastron, suddenly grows narrow 

 much faster, leaving the hind legs almost entirely exposed, but covering the 

 sacrum with a nari'ow arch, and coming to a point over the tail. In front also, 

 from its union with the plastron forward, the carapace narrows fast, but its front 

 end is truncated; the margin of the sides and end of this narrowed part, which 

 is turned rather sharply downward, are deeply concave, leaving the shoulders 

 and neck much exjjosed. The plastron narrows constantly from where it first 

 unites with the carapace to where it again separates from it, then narrowing still 

 faster it comes to a point under the pelvis, leaving the hind legs and tail 

 entirely exposed from below. It reaches forward, between the front limbs, but a 

 short distance, and is there mucli narrowed ; the front end of this narrowed 

 part is nearly straight, but the sides are concave. Thus, the hard dermal shield ^ 



a sub-family under the name of Sphargidina (Saggio authority untlcr wliich it shall be quoted henceforth. 



An. Vert. 1831.) The name of Sphargida; having My opinion is, that, in spite of Gray himself, it should 



the priority as a family name, though it is now re- be referred to as Si'iiAROiDin.E, Gray ; notwiili- 



jected by its own author, there arises an interesting standing even the alteration in tlie spelling. 



question of nomenclature in this case, respecting the * See Chap. 1, Sect. 5, p. 263. 



41 



