302 CRETACEOUS GASTROPODA 
there are among them species to be found which still possess rudiments of gills, 
such a case would immediately remove all the doubts which can at present be 
raised against the classification of the two families in one division. The name 
PLANILABIATA which we have suggested would be characteristic for both; and a 
few other allied forms which are for the present classed in the NEUROBRANCHIA 
probably could be placed in the same tribe. 
The Neriripx are partially marine and littoral, partially brackish- and fresh- 
water inhabitants. The marine species generally have a thicker shell than the 
others, but the general form of both is very much alike. 
Fossil species of Nzzrirrpz occur all through the sedimentary formations, but 
they are never very numerously represented, as compared, for instance, with species 
of Narrcipz or Trocuip#. With reference to the small number of paleozoic 
species, Deshayes (Paris foss., 2d. edit., Vol. ITT, p. 12) correctly remarks, that they 
require to be carefully re-examined, to render their generic determination 
indisputable. It is, indeed, very much to be regretted that so little attention is occa- 
sionally paid to the form of the aperture, especially to that of the inner lip; and 
that specimens are simply determined as Nerite from an external resemblance 
of form. There is very little to be learned from a figure which is merely a repre- 
sentation of the back-view of a specimen; and still such so-called species are 
often successively transferred to half a dozen different genera, without the slightest 
further contribution towards a knowledge of their generic characters. 
True species of Nzrirm are known from the Trias, and from that time their 
number gradually increases in the successive secondary strata, until they reach 
their maximum in the present time. It is remarkable that most of the oldest, 
triassic species are smooth, like Neritine, though they always have the thick 
shell of Nerita. 
There has already been so much written about the generic or sub-generic 
distinctions in the family Nzzrrrpz, that we hardly need to repeat here the con- 
troversies which are to be found in the different treatises on conchology. So far 
as concerns the specific forms, which are pretty well known, there cannot be a 
doubt that it is very desirable to make certain generic distinctions among them. 
The two groups of shells, usually quoted under the names of Nerita and Neritina, 
can generally be separated without difficulty. To determine, however, the other 
genera correctly we still very much require a knowledge of the organisation of the 
animals. It is simply impossible to imagine, that identically organised Molluscs 
produce a different structure of the external shell; for if these differences origin- 
ated in a change of their habits and manners of life, they would only prove that 
the influence of external agencies has already effected a change in their organi- 
sation. Unless, however, we distinctly point out the organs which produce these 
structural changes of shell, and what value may be attached to them in the 
classification, we cannot expect our generic and sub-generic names to be of any 
permanent use. <A trinominal nomenclature would very much assist us in such 
cases. Excepting in the Amwoyrrms, there are not many other instances, 
