SEMISINUS.—NERITINA, 465 
S. kochi, Bernardi, Journ. de Conch. v. t. 3. fig. 6 (1856); Reeve, loc. cit. fig. 21; and 
Brot, loc. cit. p. 385, t. 40. fig. 4, from Brazil (and, according to H. v. Ihering, also 
from Eastern Peru), is allied to this species, but it has the last whorl more ventricose 
and longer in regard to the whole length of the shell. 
NERITINA. 
Neritina, Lamarck, Philosophie Zool. i. p. 321 (1809) ; Hist. Nat. Anim. sans Vert. éd. 1, vi. 2, 
p- 183, éd. 2, vill. p. 564; Sowerby, Conch. Illustr. parts 86 &c. (circa 1838) ; Thes. Conch. 
i. p. 507 (1849) ; Récluz, in Journ. de Conch. i. pp. 148-154 (1850) ; v. Martens, in 
Martini & Chemnitz, Syst. Conch.-Cab. ed. 2, ii. part 10 (1875-79) ; Fischer & Crosse, 
Miss. Scient. Mex., Moll. ii. p. 4.56. 
Shell semiglobose or globose, with few whorls ; aperture large, semicircular ; 
columellar margin straight, cutting, continued outside in a distinct plane (area), formed 
by a glossy peculiarly coloured shelly deposit, the margin itself usually finely toothed, 
seldom smooth, the outer margin of the aperture not toothed within. The exterior of 
the shell usually covered with a dark greenish or blackish periostracum, generally 
smooth. 
Operculum oblong-semicircular, calcareous, with a membranaceous edge at the 
external convex border; two projecting processes (apophyses) within, one at the 
lower end of the operculum, straight, somewhat oblique, usually stout, but in 
some of the subdivisions of the genus scarcely or not at all developed, the other 
arcuated, compressed, encircling the former. 
Two long filiform feelers, the eyes on tubercles at their outer base. 
-Radula rhipidoglossate. 
Most of the species living in fresh water, running or stagnant, some also in brackish 
or salt water, crawling on stones or other solid objects. Oviparous; several eggs in a 
convex, membranaceous, pale yellow, pouch-shaped egg-capsule (ovicapsula), which is 
attached to the outside of other shells of the same species or to those of other genera, 
e. g. to Pachychilus, which live in the same water. 
The genus Neritina may be subdivided as follows :— 
A. Nerira, Roth. 
Both apophyses of the operculum well developed and separated from one another. 
Columellar margia denticulated, not peculiarly sinuated. 
To this subgenus belong the majority of the Mexican and Central-American species, 
especially all true freshwater forms. They can be separated into three groups, 
according to the general form of the shell :— 
BIOL. CENTR.-AMER., Terr. and Fluviat. Mollusca, November 1899. 59 
