272 SHELLS AND SHELL-PISH. PART I. 



by a peculiarity in the construction of the animals, first 

 ascertained, we believe, by Mr. Lea. Here, then, we 

 have two obvious modifications of semicircular warted 

 shells the one winged, the other not. This rounded 

 shape, however, begins to be lost in succeeding species, 

 and we are thus conducted to that singular shell the 

 Unio cylindricus of Say (c). Notwithstanding its un- 

 common length, it yet preserves, in every other part of 

 its outward appearance, such a close affinity to metanevra 

 (6), that, without knowing why, a conchologist would 

 at once place them together. On turning to plicata 

 we see another modification : this species is neither semi- 

 circular, like metanevra, nor elongated, like cylindrica; 

 it is broadly oval ; and, instead of being tuberculated, has 

 merely two or three deep oblique furrows : it exhibits, 

 in short, but one out of the three typical characters, 

 namely, the winged elevation of the posterior angle. 

 We believe more than two or three species pass under 

 the name of plicata. In one of these, kindly sent us 

 by our liberal friend Mr. Cooper, of New York, and 

 labelled as " a very old specimen," there is but one 

 lateral tooth on the right valve, instead of two ; so that 

 this species or variety actually puts on the very cha- 

 racter of our Megadomus gigas *, and renders both proto- 

 types of Alasmodon. But has Nature no method in these 

 remarkable variations ? Can it be supposed that the devi- 

 ation of species, or of races of the same species, are 

 regulated by no fixed principles, while all the- rest of 

 creation, inconceivably diversified, has been created on 

 one and the same plan ? Every analogy of reasoning, 

 every ascertained fact, is against the supposition. 

 Whence, then, does the species before us put on the very 

 aspect of another genus ? Simply for this reason, that 

 plicata, and its immediate allies, represent the genus 

 Alasmodon, and that Nature adopts this mode of in- 

 structing us in her favourite theory of representation. 

 With plicata must be associated our Unio rugosus, 



* It is this fact which so strongly leads me to suspect that there may be 

 some error in our location of Potomida. 



