OF SOUTHERN INDIA. 331 
semi-circular, outer lip sharpened from within, inner lip usually with a toothed or 
plicated edge. Operculum shelly, subspiral, with a process on the inner edge. 
All the Nzzrrrpxm absorb the internal portions of the upper whorls, either 
partially, or wholly, producing thus a simple cavity which occupies the top of 
the shell. This is sometimes important in the determinations of cast specimens, 
which can thus be readily distinguished from those of the Naricrp# by the want 
of any spiral apex. 
The animals of the Nzzrzrzp# are distinguished by the great length of their 
setaceous tentacles and by their large triangular or roundish foot, which has no 
lateral fringe-appendages, nor any kind of filaments; the head-or neck- lobes 
are also wanting. ‘The rostrum is short, broad and flattened ; the radula very long, 
the middle teeth unequal, the second and fifth on each side being much larger, 
than the central and the third and fourth lateral ones; the other lateral teeth are 
uniform, thin, very numerous and hooked at the tip. 
It is remarkable how very much the animals of the Nzzriripz resemble those 
of many Hzzicrvip#, and the dentition so far agrees in both, that the lateral teeth 
are very numerous and uniform, but the central ones of the latter are very different 
from those of the former. The form of the shells, with their non-umbilicated 
and flattened inner lips, is also similar in both families. Troschel and other 
conchologists classify, therefore, the Hzzicrvipz next to the Nzzzrm.z, in the 
sub-order RutprmpociossA. We have for the present no materials either to confirm 
or to oppose this classification. If, however, the dentition is to be considered as 
the principal basis of a higher classification, the place assigned to the Hzzicrnipz 
by Troschel has good grounds. It is by no means very improbable, that the 
Hexicrnipz represent by a series of more highly developed forms that group of 
shells to which also the Nzrrrrpz belong. Thus, comparing the marine and fresh- 
water species of the Nzririp# and those of the Hzzrorrpz we have only a repe- 
tition of the same, or very similar, structural transformations or changes in the 
shells and animals, as I have previously noticed in the Rissorp#. I then stated 
that the study of such gradual changes in the organization of the animals,— and in 
their shells as connected with this,— is the only way in which we can hope to arrive 
at a natural system of Mollusca. It is no doubt quite evident, that in tracing 
out these natural groups of shells, the distinctions based upon the differences in 
the respiratory organs must become of subordinate importance. But on the other 
hand it is as yet not proved, whether the form of the dentition will offer us a 
better and more reliable basis for classification. It is well known, that the dentition 
is in some cases very variable among similar species, while in others it appears to 
be very constant. Every new discovery shows, that we must base our classification 
upon a summary of characters, and must as much as possible avoid giving to single 
distinctive characters a general meaning. 
It is known that the respiratory cavity is placed on the back of the neck 
exactly similarly in the Nzzzrrp# and in the Hzzrcryip“. Supposing, for instance, 
that further examinations of the animals of the Hzzzcryrpz should show, that 
