104 AMERICAN JOURNAL 



part of the aperture, and from Melania in having a callous 

 columella. 



The operculum has the nucleus subcentral, and is composed 

 of two or three spiral revolutions. 



duhiosus, Say. femigineus, Lea. simplex, Say.* 

 "Subgenus Potadoma, Swainson, (i. 299.) 

 Shell ovate, solid ; spire short, whorls smooth ; inner lip 

 somewhat thickened ; aperture produc^:d in front; outer lip 

 acute, simple. 



depygis, Say. ovoideus, Lea. 



gracilis, Lea. rufescens. Lea. 



inornatns, Anth. sordidus, Lea. 



Isevigatus, Lea. sulcylindraceus, Lea. 



Niagarensis, Lea. suhsolidus, Lea. 



Ocoeensis^ Lea. Warderianns, Lea.f 



" Genus lo, Lea, (i. p. 299.) 



Shell subfusiform, whorls spinose; aperture large, ovate, di- 

 lated anteriorly, produced in front into a grooved beak ; outer 

 lip simple, acute. 



Syn. Melafusris^ Swains., GlotelJa, Gray. 

 Ex. I. fluviatilis, Say, t. 31, f. 8. Operculum, f. 8, a, h. 

 The species of /o inhabit the rivers of North America; the 

 shells, like those of most of the Melaniidse, are covered with a 

 brown, black, or olivaceous epidermis, and are remarkable for 

 the peculiar elongation of the axis anteriorly, and for the spi- 

 nose nature of the last whorl. . 



armigera. Lea. pernodosa. Lea. 



DuUoniana, Lea. plicata, Lea. 



Florentiana, Lea. robulina, Anthony. 



fluviatilis, Say. spinigera, Lea. 



fustformis^ Say. spinosa, Lea. 



nobilis^ Lea. tenehrosa, Lea. 



pagodula, Gld. tuberculata, Lea.:{: 



" Subgen. Elimia, H. & A. Adams, (i. p. 300.) 

 Shell fusiformly ovate ; whorls reticulate or nodulose, carinate 

 in the middle ; aperture greatly produced anteriorly ; outer lip 

 thin, simple, acute. 



* The genus PaeJiycJieilus ^ys.s instituted by Mr. Lea to comprise a cer- 

 tain form of sliclls attaining tlieir greatest numerical development in Cen- 

 tral America. There are no shells inhabiting the United States which arc 

 congeneric with these ; and Messrs. Adams have entirely mistaken the 

 scope of the genus in including such species as simplex. 



I = simplex, Say, which Messrs. Adams place in the genus Pachycheilus 

 as typical ! 



:[: Among the species here ev.vim.eraXe(S.diYe Angitremm, Anculoscr, Lithasice, 

 Strephobases, Goniobases, and Pleurocerce. I. pagodula is an exotic species, 

 and does not belong to the genus. 



