Xlll 



The operculum has the nucleus subcentral, and is composed 

 of two or three spiral revolutions. 



dubiosus, Say. fernigineus, Lea. simplex, Say.* 

 " Subgenus Potadoma, Swainson (i, 299.) 

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

 somewhat thickened ; aperture produced in front ; outer lip 

 acute, simple. 



depygis, Say. ovoideus, Lea. 



g}-acilis, Lea. riifescens, Lea. 



inornatus^ Anth. sordkhis, Lea. 



Icevigatiis, Lea. stibcylindixiceus, Lea. 



I^iagarensis, Lea. svhsoUdus, Lea. 



Ocoeensis, Lea. Wardericmus, Lea.f 



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



Shell subfusiform, whorls spinose ; aperture large, ovate, 

 dilated anteriorly, produced in front into a grooved beak ; 

 outer lip simple, acute. 



Syn. 3fekifusHs, Swains., Glotella, Gray. 

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

 The species of lo inhabit the rivers of North America ; the 

 shells, like those of most of the JlelanikUr, 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. 



Duttonkina, Lea. pUcata, Lea. 



Florentkow, Lea. rohuUna, Anthony. 



JliivkitiUs, Say. sjnnigera, Lea. 



fusiformis, Say. spinosa. Lea. 



noblh's, Lea. tenebrosa, Lea. 



2X(goduki, Gld. tuberadata, Lea. | 



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



Shell fusiformh' ovate ; whorls reticulate or nodulose, cari- 

 nate in the middle ; aperture greatly produced anteriorly ; outer 

 lip thin, simple, acute. 



* The conns Pachycheilus was instituted by Mr. Lea to comprise a 

 certain form of shells attaining tlieir greatest numerical development 

 in Central America. There are no shells inhabiting: the United Slates 

 which are congeneric with these; and Messrs. Adams have entirely 

 mistaken the scope of the {rcnus in including such species as simplex. 



t = simplex, Say, which Messrs. Adams place in the genus Faclitj- 

 cheihis as typical ! 



X Among the species here enumerated are Angitremce, Anculosa:, 

 Lithasice, Strephobases, Goniobnsr.s. (in(\ Pieyrorera;. I. }^a godnla is ao 

 exotic species, and does not belong to the geuus- 



