FAMILY 3. NERITACEA. 133 
were, the straight side of a semicircular aperture ; De Blainville has altered 
the name of Neritacea to that of Hemicyclostomata in order to distinguish 
this peculiarity of form in the aperture ; but we shall ever remain opposed 
to these unnecessary changes in the nomenclature. 
The following are the five genera into which this family is divided : 
FLUVIATILIA: Marina: 
NAVICELLA. NeErIrTA. 
NERITINA. NERITOPSIS. 
NatTICca. 
NAVICELLA, Lamarck. 
Testa transversim elliptica, vel oblonga, patelleformis, superné convexa, 
epidermide olivacead induta; spira nulla; vertice ad marginem sub- 
oblique inflexo; labro integro, complanato, superné columellam 
tenuissimam formante ; apertura amplissima, impressionibus mus- 
cularibus duabus distincté imbuta. Operculum testaceum, subqua- 
dratum, solidum, planum, dente laterali, acuto, instructum. 
The genus Navicella was introduced by Lamarck in order to distinguish 
a small group of fluviatile mollusks that had been associated by Gmelin, 
De Roissy, and others with the Crepidule. They were assigned to that 
genus by those authors on account of the crepiduliform convexity of their 
shells, without, perhaps, being aware of the important difference in their 
habits: this was, however, detected by Lamarck, and the same obser- 
vations were simultaneously made both by De Montford and De Férus- 
sac ; for whilst they were separated from the Crepidule by the former for 
the formation of his genus Cimber, the latter proposed to distinguish them 
with the new generic appellation of Septaria. De Férussac still conti- 
nued to follow, to a certain extent, the arrangement of his predecessors 
in placing the Navicelle in the same family with the Crepidule ; so also 
