268 



SHELLS AND SHELL-FISH. 



found in any other of these groups* (T. metanevra 

 Rsif. fig. 50.). 5. Potomida: the shell rather short 

 and oval ; the umbones prominent, but not so large as 

 in Unio ; the outer surface smooth ; the cardinal teeth 

 thick, perpendicular, but the outer one diverging. ^These 

 are mostly European shells, and are represented by U. si- 

 niiata Lam., and our P. corrugata {fig. 51.). Between 

 51 these sub-genera are in- 

 tervening species, or 

 rather types of form, 

 which blend the whole 

 into a continuous chain 

 of affinity ; thus P. si- 

 nuata is so like some of 

 the typical Unios, that 

 the direction of the outer 

 cardinal tooth and the comparative smallness of the bosses 

 alone distinguish them. Leaving, however, these inter- 

 vening or connectant species, and looking only to the 

 types, we shall see by the following table that they 

 have a resemblance to the primary divisions of the 

 whole family, and to the genera of the UnionincB, 



Analogies of the Sub-genera of Unio. 



Sub-genera 

 of 



Unio. 



Analogies. 

 ^ Shell ovate ; cardinal teeth 1 



Sub-families 



of 



Vnionidce. 



Unionin^. 



t not diverging. 



("Shell more oval, wedge- 7 «,.„,„ „ 



X shaped, and angulated. J">RIN^. 



Shell very much lengthened. laiDiNiE. 



Cunicula. 



Ligutnea. 



T....-..r^a.[P-\erio^rMnge margin gene-|^^,„^^,^^ 



("General form oval, oblong:") 



Genera 



of 



Unionitus. 



Unio. 



^GLIA. 



Mysca. 

 Lymnadia. 



Potomida. < cardinal teeth obtuse, 

 C verging, sometimes erect 



Alasmodoxin'jj. Megado.mus. 



The analogy of the last three groups is not so satis- 

 factory as the others, and this excites a suspicion that 



* Except U. cornutus and JEsopus. U. nodosa. Wood's Conch, pi. 22. fig. 

 1, 2., seems to belong to this group. 



