280 SHELLS AND SIIELL-FISH. PART I. 



fragiUs, two. Now^ when we reflect how many more may 

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

 bers being midiscovered^ 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. 



(2()2.) 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; w'hile the 

 other, as Lamarck, Say, and Barnes have correctly 

 judged, is one of absolute aflSnity : 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 ]\Ir. 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. 



