OF SOUTHERN INDIA. 333 
or examples known in which conchologists would better see the great advantage 
deriveable from such a trinominal designation ; or in other words, from the proper 
use of sub-generic distinctions. The natural grouping of shells would certainly 
derive from its use greater benefits, than could be counterbalanced by the slight 
inconvenience which it would possess as compared with the binominal system. 
We give here a short summary of the different generic and sub-generic forms 
known up to the present. 
1. Navicella, Lamarck, 1809, (Catillus, Humphrey apud H. and A. Adams, 
Gen. I, p. 386; Gray, Guide, 1857, p. 189,) is the patelliform representant of the 
family ; only one or two sub-fossil species have as yet been reported. 
_ 2. Pileolus, Cookson, 1823, is known only in a fossil state. The secondary 
species have the basis generally rounded and the apex subcentral, while the tertiary 
species have it more oval and the apex terminal. ‘These last named species 
approach, therefore, pretty nearly the recent Mavicelle, and Deshayes remarks that 
the name 
2a. Tomostoma, which he proposed in 1823 (publ., 1824) might be applied to 
these tertiary species (vide Paris foss. 2d. ed., Vol. IIT, p. 25). 
3. Neritina, Lamarck, 1809. (Neritella, Humphrey in H. and A. Adams’ 
Gen. I, p. 380; Gray’ Cat., p. 187). Nearly all Neritine live in brackish - or fresh - 
water, and are generally covered with a horny epidermis. The typical species have 
a distinct, shortly conical spire, the shell is thick and globose, the edge of the inner 
lip insinuated, more or less distinctly denticulated. 
The following forms have been distinguished as sub-genera :— 
3a. Velates, Montfort, 1810 (vide Chenu’s Man. I, p. 337). According to 
Deshayes the spire is distinctly traceable in young specimens, and becomes enve- 
loped with callosity only in later stages. The typical species, Vel. Schmideliana, is 
distinguished by its broadly conical form, large extent of the inner lip, and its 
strong denticulation along the entire inner edge of the aperture. 
3b. Neripteron, Lesson, 1830, has the external edges of the apertural margin 
posteriorly and anteriorly much expanded; the form is depressed with a sub- 
posterior and lateral apex; edge of the inner lip denticulated. 
3c. Alina, Recluz, 1842; in form like the last, but the margin has only the 
upper or posterior external edge expanded. 
3d. Dostia, Gray, 1840. Shell oval, depressed with lateral incurved apex. If 
Humphrey’s name Neritella has any right to be accepted, it could apply only to 
this form. 
3e. Vitta, Klein, 1753. Shell transversally oblong with a short indistinct 
spire, slightly or obsoletely toothed on the edge of the inner lip. 
3f. Clithon, Montfort, 1810 (H. and A, Adams, Gen. I, p. 384,) includes a 
number of species which usually have some kind of spines or tubercles on the 
posterior portion of the whorls. These spines generally only develope in old 
specimens, being often wanting in the young stages. A more useful characteristic of 
Clithon is the invariable presence of a strong fold-like tooth on the inner lip, being 
4 N 
