UNIONIN^, ANALOGIES. 



267 



these genera. Two, however, where the natural series 

 appear to us very strikingly marked, will be now 

 noticed : the first regards the sub-genera of our genus 

 Unio, and the circular succession of the types of form 

 in two of them. 



(252.) The genus Unio^ separated by its cardinal 

 teeth from ^glia, as already shown (^figA^.h), contains 

 five distinct types of form, which, from the multiplicity 

 of species, and for the greater facility of nomenclature, 

 we shall designate under the following sub-generic 

 names: — 1. Unio: the bosses or umbones prominent 

 and tumid, the circumference and the posterior hinge mar- 

 gin never angulated, and both the cardinal teeth very much 

 receding from the anterior margin, as in U. Mytilo'ides 

 Raf.j and all such species or varieties. 2. Cunicula, where 

 the umbones are even larger, but not tumid, being, as it 

 were, compressed or flattened : the shell is generally 

 oval, or angulated at the base of the posterior side ; — 

 examples, C planulata, cuneata, purpurata, Lam., &c. : 

 the outer cardinal tooth slightly diverges from the base 

 of the inner. 3. Ligumia : the 

 form is particularly long, like 

 that of Iridina, the umbones 

 small and retuse, and the car- 

 dinal teeth close to the ante- 

 rior end ; as in U. recta Lam. 

 4. Thelidei'ma : the form is 

 various, but the posterior hinge 

 margin is generally elevated 

 and angulated, and the outside 

 of the valves tuberculated or 

 granulated, — a character not 



distinct species sent from that country, evidently labelled by the same hand, 

 yet bearing the same specific name. Mr. Lea's Synopsis leaves this part 

 of the subject almost as obsure as ever, because he merely cites the 7iame, 

 without giving any reference to the ivorks of the authors he quotes. By 

 his plan, as novel as it is detrimental to science and all critical inquiry, 

 the author (no doubt, unintentionally) imposes an effectual bar to the ve- 

 rification of his own nomenclature, no one here being acquainted with all 

 the detached papers on these shells printed in America. 



