CHAP. X. UNIONIN^. MYSCA. 279 



of the group : with mnrginalis must be joined three or 

 four other species, all of which are from China and 

 India. One of these now before us, bears such a close 

 resemblance to the Unio nasutus of Say. that, if the teeth 

 are not examined, it would be impossible to define their 

 difference. Nasutus, however, represents another type, 

 which at present is almost exclusively confined to North 

 America. Here the cardinal teeth greatly resemble those 

 of Say's cario^rt and radiata (which come into the circle of 

 jSIglia) ; for the outer one is somewhat conic and erect, 

 while there is only a single defined tooth in the left valve. 

 From nasuta we pass, by several intervening species, 

 into the next type, represented by onr M.Nilotica, shaped 

 like a small Anodon, but with the teeth of Mysca. We 

 possess three other species, all from the river Nile, and 

 believe there are many others. Finally, we have from 

 America the Unio fragilis, figured very accurately in our 

 Zool. Illust. 1st Series, pi. 17 1^ I'ut which the Ameri- 

 can conchologists do not appear to understand : it truly 

 belongs to their genus Unio, because the two lateral teeth 

 are well defined, although the cardinal are but slightly 

 developed. Our figure, nevertheless, was drawn from a 

 very old specimen, which did not exhibit the elevated and 

 angulated posterior margin. Now, of all the UnionidcB 

 so constructed, this makes the nearest approach to the 

 well-known Unio alata of Lamarck ; while, in the imper- 

 fection of its teeth, it clearly represents Alasmodon. 

 We are thus brought round to the group with which we 

 commenced ; and were it necessary to point out the 

 precise point where the circle is closed, Ave could expa- 

 tiate upon the structure of our M.Mexicana, — an unde- 

 scribed species, and standing intermediate between the 

 first type and the last. It must not be supposed that the 

 modifications of form represented by the five species 

 named in the last table, are merely confined to one or two 

 examples ; even our own collection of specimens and of 

 drawings can produce several, Ovata, for instance, 

 represents a little group of eight species ; marginalis, 

 another of four; nasuta, seven; JVilotica, four; and 



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