18 NERITA. 



Family NERITIDjE. 



Geuus NERITA, Adanson. 1757. 



Dr. vou Martens, in the prefoceto his monograph of Nerita, now 

 in course of publication, thus divides the geuus into groups, which 

 he considers " more or less natural," whilst not so sharply distin- 

 guished one from another as the similar divisions of Neritina. I 

 find the older and more simple divisions W'hich I have adopted full 

 of difficulties to the systematist, which Avould be much increased by 

 a more elaborate system. It is noticeable that Dr. von Martens 

 has not arranged his own monograph by the system he proposes. 

 His groups are : — 



1. Theliostyla, Morch. Whorls flattened, strongly ribbed, 

 columellar area granular, the marginal teeth mostly small. Oper- 

 culum granular. N. textilis. Natere, Gray, is a synonym. 



2. Cymostyla, Martens. With weaker spiral ribs, whorls 

 rounder, columellar area with parallel plications, the teeth stronger. 

 Operculum granular. N. undata. Plia, Mtirch, in part, but not 

 Pila, Klein. 



3. Pila, Klein. Shell bullet-shaped, with strong rounded spiral 

 ribs, and strong columellar teeth. Operculum concave, weakly 

 granular or smooth. N. plicata. Ritena, Gray, and Tenare, 

 Troschel (not Gray), are synonyms. 



4. Tenare, Gray. Teeth of the interior of the outer liji vanish- 

 ing. Oper(ailum smooth, with polished marginal zone. / 



a. Peloronta, Troschel. With weak spiral ribs. Marginal 

 zone of the operculum swollen, distinct. N. peloronta. 



h. Ilynerita, Martens. Spiral sculpture stronger, teeth of the 

 mouth weaker. Operculum with scarcely developed marginal zone. 



N. PLANOSPIRA. 



5. Nerita (restricted). Spiral sculpture weak or Avanting, colum- 

 ellar area swollen, smooth. Operculum flat, with ribbed margin. 

 N. POLiTA. Odontostoma, Morch (in part), is a synonym, 



6. Amphinerita, Martens. Shell as in preceding group. OjDer- 

 culum granular. N. Umlaasiana, N. Senegalensis, etc. 



7. Heminerita, Martens. Outer lip not toothed, columellar 

 margin likewise toothless. Operculum with strongly developed 

 skinny projection on its convex margin. N. pica. This group is 

 the most nearly related to Neritina. 



