268 



SHELLS AND SHELL-FISH. 



PART I. 



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

 Raf. 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- 

 nuata 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 Unionince. 



Analogies of the Sub-genera of UNIO. 



Sub-genera 

 of 



Unio. 



Unio. 



Analogies. 



Sub-families 



of 

 Unionidte. 



Genera 



of 

 Unionints. 



UNIO. 



Ligumea. Shell very much lengthened. IRIDIN^E. 



MYSCA. 

 LYMNADIA. 



T General form oval, oblong;} 



Potomida. } cardinal teeth obtuse, di- > ALASMODONINJE. MEGADOMUS. 

 C verging, sometimes erect. J 



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 Msopus. U. nodosa, Wood's Conch, pi. 22. fig. 

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



