468 MOLLUSCA. 
a. Autna, Récluz (1842) (part.) [SemicircuLatz, v. Martens]. 
Shell somewhat depressed, with short spire and very large columellar plane. 
Peculiar to America and Western Africa. N. latissima and N. punctulata; N. turbida 
forms a connecting-link with the following. 
5. Nerirma, Roth, s. str. [Serrarx, Recluz (1841) ; Picrw, Menke (1880) ]. 
Shell globose or conoidal-globose, handsomely marked ; columellar plane not so large, often 
convex ; toothlets of the columellar margin well developed, nearly equal. 
Circumtropical. N. lineolata, N. virginea, and N. pulchra. 
ec. Pupzrita, Gray (1857) [VEenosa#, v. Martens]. 
Shell transversely elliptic, with nearly flat spire; not shining. Mostly marine. 
Circumtropical. N. pupa. 
B. CurtHon, Montf. 
Both apophyses of the operculum well developed and connected by an intermediate wall. 
Columellar margin sinuate at its middle, distinctly denticulated, the toothlet at the upper end of the sinus 
the largest. 
Circumtropical. NV. picta (and NV. michaudi). 
C. THEopoxvs, Montf. 
The lower apophysis (peg) obsolete or absent, the other (rib) well developed. 
Columellar margin not denticulated. 
Chiefly European, and im the adjacent parts of Asia and Africa. (J. sargi and 
N. mexicana, both very doubtful as regards their habitat.) 
D. Smaragept, Issel. 
Apophyses as in A. Neritewa. 
Shell transversely oval, depressed, glossy, clear coloured. 
For the differences in the radula see Troschel. 
Truly marine, in the subtropical and tropical seas of both hemispheres. I. viridis. 
In these subdivisions and in the preceding table all the species are included, whether 
they live in fresh or salt water. But in the following pages only those species are 
enumerated which are actually found in fresh water; the others will be placed at the 
end of the volume with the brackish-water Mollusca. 
In the comparative table the height of the aperture is purposely omitted, because 
this can be measured in various ways, as the shell is held more or less obliquely, and 
it is in this genus often rather difficult to say which is the true vertical position of the 
shell; therefore the length of the columellar margin within the aperture is given 
instead, this being always considerably less than the height of the aperture itself, but 
affords at all events a certain measurement. ‘The diameter major gives the breadth of 
the shell, and the diameter minor the amount of convexity. 
