408 AMERICAN TESTUDINATA. Paet II. 



into the white gicles of the neck. A straight black hne extends in front of the 

 eyes across the space which separates them, and forms a triangle with two sim- 

 ilar lines extending from each eye to the tip of the proboscis. The largest speci- 

 men I have seen, measured twelve inches from end to end of the carapace, and 

 nine and a half across the middle. All the specimens I have examined thus far 

 were obtained in Texas. Rev. Edward Fontaine, of Austin, Texas, writes, me 

 that it delights in clear, bold, and rocky streams, and possesses nothing of the 

 sluggishness of other Testudinata, but is brisk and vivacious in all its movements, 

 running rapidly on land when dropped from the hook of the angler, and swim- 

 ming with great velocity. 



I expect to be gravely criticized for describing the species of our Trionychidae 

 in the manner in which it has been done in the preceding pages. Seeming dis- 

 crepancies may, indeed, be noticed between the generic and specific characters of 

 these Turtles as expressed here, and the description of the family characters as 

 presented in a former section. But Animal Morphology has still more striking 

 contradictions in store in its nomenclature, than those of which I may have been 

 thus far guilty. So long as our language has not yielded to the necessities of the 

 case, there will be something awkward in the use of expressions that are famil- 

 iarly employed to designate definite forms, when transferred, with quahfications, 

 to animal forms, which have neither the definiteness nor the regularity of mathe- 

 matical figures. It may appear absurd to speak of a flattened sphere, of an 

 elongated circle, (not an ellipse,) and the like ; but I hold that it is better to 

 make such a use of these words than to avoid apparent contradictions by the 

 introduction of circumlocutions ; for such expressions are at once characteristic, and 

 may become quite picturesque when judiciously applied. The family of Naiades 

 among Acephala has afforded me a welcome opportunity to test the importance 

 of form, as the leading character of families. There is scarcely another natural 

 group which embraces species apparently more diversified in their forms than 

 these shells. We need only compare Unio stegarius with U. rectus or Shepardi- 

 anus, or U. alatus with U. cylindricus, or with U. Cardium or U. torsus or U. 

 mytiloides, triqueter, flexuosus, etc. Every possible form seems to be represented 

 in that family, from the quadrangular or triangular to the spherical. And yet 

 all Naiades have one and the same typical form, determined by their internal 

 structure, wliich may be described as ovate, with a double flexure on the lower 

 side, towards the hind extremity; and this form is determined by the structure of 

 the mantle.^ Unio flexuosus exhibits this typical form in its most distinct out- 



^ I shall have an opportunity to illustrate these the fifth, which is to be devoted exclusively to the 

 statements most fully in a future volume, probably history of our fresh-water Mussels. 



