108 PALAEONTOLOGY OF THE TJPPEE MISSOURI. 



SUBCLASS PROSOBRANCHIATA. 



ORDER Riphidoglossata. 



SUBOKDER PODOPTHALMA. 



FAMILY NERITWM. 



Shell generally thick, varying from subovate or ovate-subglobose, to 

 depressed hemispherical, not umbilicate ; spire very small or sometimes 

 nearly obsolete, often lateral, simple within, from the absorption of the 

 inner whorls ; body volution very large ; aperture semilunar, not pearly 

 within. 



Operculum wholly, or in part shelly, subspiral, articulated with the 

 inner lip by one or two processes. 



Animal without lobes or neck lapets ; muzzle broad, short, and more 

 or less emarginate ; tentacles long, subulate, and having the eyes on pe- 

 duncles at their outer posterior bases. Foot oblong-subtrigonal ; margins 

 simple, not provided with filaments or membrane. Dentition much as in 

 the Trochidce (x.3, 1, 3.x), the rachidian tooth being very small, laterals 

 unequal, and the uncini numerous, with the first one large, and the 

 others very small, slender, and hooked. 



This family embraces a large number of species, which have been variously 

 grouped into genera and subgenera by different authors. The groups most usually 

 adopted for the reception of the recent species are Nerita, Neritella, Clithon, Alina, 

 Neripterov, and Catillus. The extinct genera known at this time are Neridomus, 

 Velates, Deshayesia, Neritoma, and Pileolus. There are, however, probably several 

 undescribed genera amongst the fossil species referred to Nerita; and some of the 

 Carboniferous shells referred to McCoy's genus Naticopsis (though not the typical 

 species) seem to belong to some genus of this family rather than to the Naticidce. 



Genus NERITELLA, HUMPHREY. 



Synon. Neritella, HUMPHREY, Mus. Col. 1797, 57. GRAY, Zool. Proc. 1847, 148. H. & A. ADAMS, Genera Recent 

 Mol. I, 18C-4, 380. 



NerMna, LAMK. Phil. Zool. 1809; Hist. VI, 1822, 182. BRONN, Leth. 1837,390 PDSCH, Pol. Palaeont. 

 1837, 97. SWAINSON, Malacol. 1840, 347. RECLUZ, Revue Zool. 1841, 273, &c. 



Theodoxus, MONTFORT, Coneh. Syst. II, 1810, 351. 



Lamprostoma, RAFINESQDE, Anal. Nat. 1815 (not SWAINSON, 1840) 



Nereina, DE CRIST, and JAN. Cat. 1832. 



Clypeolum, RECLCZ, Revue Zool. 1842, 234. 



Puperita, GRAY, Guide to the Syst. Dist. Mol. 1857, 137. 

 Elym. Nerita dimin. 

 Type. Nerita viridis, LIN. 



Shell ovate, or rhombic subglobose, rather thin, covered with a corneous epider- 

 mis ; surface smooth or striate, and often ornamented with beautiful and vivid colors. 

 Spire short, more or less conoid. Aperture semilunar; inner lip and columella 

 straight and flattened or septiform, with a smooth or crenulated margin. 



