246 Lamarck's Genera of Shells. 



3. Ampullaria *. 



Shell globular, ventricose, umbilicate at the base ; no callus 

 on the left lip ; aperture entire, longitudinal ; margins united ; 

 right margin not reflected ; an operculum. 



The last whorl is, at least, four times as large as the penulti- 

 mate. The columellar lip projects, and is reflected over the 

 umbilicus, forming a half funnel, but no callus. The shells of 

 this genus are generally large. 



Type. Ampullaria Guyanensis t. 



Shell ventricose-globular, solid, longitudinally, and unequally 

 striated; epidermis brown ; six whorls ; last whorl the largest ; 

 aperture orange-coloured. 



Rivers of Guiana. PI. VII. Fig. 149. II species. 



5th Family. 

 Neritacea. (4 genera.) 

 Operculated trachelipoda ; some inhabit fresh water, others 

 are marine. 



Shell fresh-water or marine, semi-globular or flattened oval ; 

 no columella ; left margin of the aperture acute, transverse, 

 and resembling a half partition, with which the operculum 



articulates. 



1. Navicellat. 



Shell elliptical or oblong, convex above ; summit straight, 

 depressed to the margin; under side concave. Left margin 

 flattened, acute, narrow, toothless, transverse § ; operculum 

 solid, flat, with an acute lateral tooth. 



Exotic, fresh-water shells ; distinguished from nerita and 

 neritina by the summit not being spirally convolute. The 

 transverse, left lip, never covers half the cavity. 

 Type. Navicella tessellata ||. 



Shell oblong elliptical, thin, diaphanous, tessellated with 

 yellowish and brown oblong square spots; vertex maginal, not 

 projecting. 



Rivers of India. PI. VIII. Fig. 150. 3 species. 



* AmpvUa, a wide-hellied bottle, f Of Guiana. + Pro navicula, a little boat. 



^ The transverse position of the flattened left lip gives the shell, when 

 held with the concave side upwards, the appearance of a little boat with 

 a half-deck. || Tessellated — 3rd Species. Lamarck's type is N. elliptica. 



