280 SHELLS AND SHELL-FISH. PART I. 



fragilis, two. Now, when we reflect how many more may 

 be in cabinets, and the probability of still greater num- 

 bers being undiscovered, we should be fully waranted in 

 designating these sections as sub-genera ; but so long as 

 any doubts remain regarding the true affinities of our 

 sub-genus Potomida, this plan may be suspended. 



(262.) The Lymnadia alata is one of the most re- 

 markable shells of this sub-family; not so much on ac- 

 count of its beauty, but as holding a more isolated station 

 than any yet noticed. That it has a strong resemblance 

 to Anodon, is indisputable ; but that it has a much 

 stronger one to the Unionince, cannot be questioned. 

 Now, every one who draws the distinction between 

 analogy and affinity, immediately perceives that the first 

 of these resemblances is purely analogical; while the 

 other, as Lamarck, Say, and Barnes have correctly 

 judged, is one of absolute affinity : it is needless, there- 

 fore, to discuss this matter further. At present, Lym- 

 nadia alata (fig. 48.) stands almost alone as the repre- 

 sentative of that sub-genus which typifies the anodons. 

 In conformity with this analogy, we find the posterior 

 dorsal margin remarkably elevated, dilated, and winged. 

 The ligament also is connate, that is, entirely covered 

 by a prolongation of the testaceous substance of the valves: 

 the bosses small, and the shell itself oval and depressed. 

 The cardinal teeth are very peculiar : they are compressed 

 and crenated like those of Mysca; but the inner one is 

 generally the largest; and the direction of the groove which 

 separates them, is either perpendicular, or diverges from 

 the anterior margin of the shell. It is clear that our Unio 

 fragilis* either enters in this group or is at the confines of 

 the last: the next question is, what others can be arranged 

 in the same genus with Lymnadia ? We entertain little 

 doubt that the Symphynota compressa of Mr. Lea is one of 

 these, f In this curious type, the cardinal and the lateral 

 teeth are perfect, and it has the complete external aspect of 



* ZooL 111. 1st Series, pi. 171. 



f We only know this from the very excellent description and figure in 

 Mr. Lea's paper. 



