132 CLASS III. GASTEROPODA. ORDER VII. PECTINIBRANCHIATA. 
Family 3. NERITACEA. 
Testa subglobosa, columella perampla, septiformi, obliqué effusa. Ani- 
mal vel fluviatile, vel marinum, operculo aut corneo, aut calcareo. 
The family of the Neritacea includes a very natural and well-defined 
assemblage of mollusks, exhibiting in their habits a transition from the 
fluviatile to the marine kinds of the class ; one portion of them are inha- 
bitants of fresh water, the other marine. They were all associated by 
Linneus in one and the same genus, Nerita, for he detected a peculiarity 
of structure in the septiform columella of their shells, which was sufficient 
in his opinion to constitute their chief generic character. The Neritacea, 
however, have not only been divided into five genera, after the manner 
here followed, but they have been assigned by Cuvier, De Férussac, 
and others to different parts of the class. The Navicelle, for example, 
were arranged by those authors in the immediate vicinity of the Crepi- 
dule, a genus of Capulacea, though it is really difficult to imagine how 
they could have been tempted to associate together two genera of mol- 
lusks differing so materially in their habits ; the one remaining adherent, 
and marine, whilst the other is fluviatile and free ; the one having a shell 
exceedingly variable both in form and structure, shaping itself to the 
irregularities of its place of attachment; whilst the shell of the other 
always exhibits a regular uniformity of growth, never varying in its 
formation, and pre-eminently distinguished in being operculated. The 
Natice, again, have been considered by some writers to be hardly sepa- 
rable from the Sigareti ; for, like those mollusks, they have a very widely- 
expanded disc, by which their shell is almost entirely enveloped. 
The Neritacea are very numerous in species, and we are much in- 
debted to M. Recluz for the elaborate manner in which the new ones 
have been lately described. 
The shell of the Neritacea, as we have already stated, is chiefly distin- 
guished by the septiform structure of the columella, which forms, as it 
