OF SOUTHERN INDIA. , 321 
distinguishable; this seems in reality to be the case, though not as yet supported 
by the organization of the animals. The genus Nacella of Schumacher is apparently 
correctly placed by the same author in the family Tzcrvrem#. The animals have the 
mouth entire, not notched, the dentition consists of six series of teeth, and the 
gills are distinctly interrupted in front of the head. The shell is ovate, conical, 
depressed and thin, more or less smooth with a sub-anterior apex and on the whole 
resembles Helcion. Gray quotes a species, Nacella mytiloides, including, however, 
others like N. cerulea, Linn., under the generic name of Patina in the family 
Paretts. There seems to be no particular difference between the two species, 
and H. and A. Adams refer them both to one genus, but place it also in the 
last named family. Thus all these and other generic distinctions are as yet so 
very unsettled, that it is most difficult to accommodate to them the numerous fossil 
species. In consequence of this uncertainty several other new genera (Zecturella 
and others) have been proposed in addition to those already existing, but they are 
equally ill-defined for the use of the paleontologist. In the greater number of 
cases indeed it solely depends upon the individual opinion of the author, what he 
supposes to be a shell of the Tzcrvrrpz rather than of the Parerrrpz. Strictly 
speaking there are no means of distinction on the shells themselves, and we must 
be guided, therefore, only by their greater or lesser resemblance to the living 
species of either of these families. Perhaps we would approach nearest to the 
truth, by arranging the fossil species according to the following distinctions,— 
a, Helcion, including forms with an oval or suboval aperture, an excentric, 
pointed and incurved apex, and the surface covered with radiating ribbings ; 
8, Nacella, similar in form to the last, but more depressed and generally smooth on 
the surface and porcellanous within; y, Zectwra including forms with a broadly ovate 
or circular aperture, apex subcentral, or somewhat anteriorly placed, pointed but 
not incurved, and the exterior of the shell smooth or covered with radiating strize 
which, when present, do not project much on the apertural margin, producing 
only a slight crenulation of the same ;—, to Patella we would finally refer all those 
species which are generally somewhat depressed, with an obtusely pointed apex and 
with strong, more or less unequal and on the margin projecting radiating ribs. 
It is of course unnecessary to state that this kind of distinction can only be 
considered approximately correct, and in fact ought to be resorted to merely in 
those cases where other and more important distinctions of the shells cannot be 
traced out. Klein in his Tent. Meth. Ostrac., p. 114, long since proposed in his 
Patelle integre and P. lacere a distinction similar to that which we have men- 
tioned as existing in general between Tectwra and Patella. 
Tn a geological point of view the Tzcrvrinz are very important shells, espe- 
cially because some of the forms externally so closely resemble fossil Capurrp2. 
They are numerous from the Silurian rocks upward all through the secondary 
and tertiary deposits, 
Pictet enumerates in his Mat. Pal. Suisse (3me. Ser. p. 717), 37 European speciés which he 
without any further distinction refers to Helcion ; there are, however, evidently species of Tectura 
and Anisomyon (LEPETID&) amongst them, and these have gradually to be separated. Of all the 
4K 
