GENERA OF SHELLS. 71 



Ampullaria. Shell globular, inflated, um- 

 bilicated at base, without callosity on the pillar 

 lip. Aperture entire, oblong ; margins united. 

 Lip sharp, not reflexed. Operculum. 



The last whorl is at least four times as large as the 

 penultimate. The coliimellar lip projects, and is re- 

 flected over the umbilicus, forming a half funnel, but 

 no callus. The shells are generally large. 



A. Guyanensis, rugosa, fasciata,canaliculata, efFusa, Guin- 

 iaca, virens, carinata, avellana, intorta, fragihs. 



NERITACEA. 



Shell fluviatile or marine, semiglohular or flattened oval, 

 without columella, the left margin of the opening resem- 

 bling a half partition. 



(1.) Fresh water shells. 



Navicella. Shell elliptic or oblong, con- 

 vex above, concave beneath ; spire straight, 

 bending over to the lip. Columella flattened, 

 sharp, narrow, toothless, almost transverse. 

 Operculum solid, flat, with a subulate lateral 

 tooth. 



Distinguished from nerita and neritina by the sum- 

 mit not being spirally convolute. The transverse left 

 lip never covers half the cavity. 



N. elliptica, lineata, tessellata. 



Neritina. SheU thin, semiglobose or oval, 

 flattened beneath, not umbilicated. Aperture 

 semicircular, piflar lip flattened and sharp : no 

 teeth nor crenulations on the internal surface 



