OF SOUTHERN INDIA. 138 
Order, PROSOBRANCHIA, Milne Edwards. 
CHARACTER.— Gastropoda with conical or spiral shells, present even in the first 
stage of growth; larve with deciduous cephalic fins; gills placed in front of the 
heart ; sexes distinct.* 
By far the larger number of the PRosoBRANCHIA are provided with an oper- 
culum, of which the structure and the position in the aperture are very important 
for all purposes of classification. Without exception, they are inhabitants of fresh 
or salt waters, although several species can, for a shorter or longer time, live outside 
of this medium in a moist atmosphere; and a few possess even an air-breathing 
organ, besides the gills. 
The PRosoBRANCHIA are proportionally more numerous than any other order of 
the Gastropoda. This fact is equally true as regards the fossil, as the living species, 
which are, so far as our knowledge extends, nearly equal in number.t 
Fossil species of PRosopraNcHIA occur from the lower Silurian upwards through 
the whole series of the sedimentary deposits. Their solid shells usually admit of a 
good preservation in the fossil state; while the fact, that they are chiefly inhabitants 
of littoral waters of moderate depth, renders the study of this order of very great 
importance to the paleontologist. A knowledge of the PRosoBRANCHIAN fauna alone 
is sometimes sufficient to enable just conclusions to be drawn, as to the conditions, 
under which the materials, which once buried and now enclose these fossil shells, have 
been deposited. 
We have already observed, that it appears desirable to separate from the order 
PROSOBRANCHIA, the PotypLAcoPHoRA and the NeuRoBRANCHIA. Excluding these, 
we shall have then to deal with the three next divisions, CTENOBRANCHIA, ASPIDO- 
BRANCHIA and CycLoBRANCHIA, of which the second may rather be replaced by 
three others proposed by Gray—ScuUTIBRANCHIA, SCHISMATOBRANCHIA and DicRANo- 
BRANCHIA. The South Indian cretaceous rocks have yielded species in nearly all 
these divisions; and, according to the arrangement already adopted, we begin with 
the highest, the 
Sub-order, Ctenobranchia. 
CHARACTER.— Prosobranchia with usually spiral shells, animal with the respira- 
tory cavity on the neck containing one well-developed comb-like gill, rarely with a 
second gill in a more or less rudimentary state: the males have usually strongly 
developed external copulative organs. 
The CrENOBRANCHIA, in this sense, comprise the greatest portion of Cuvier’s 
PECTINIBRANCHIA, and are generally divided into StpHonosToMATA and HoLosToMaATa. 
Although far from being a natural grouping, this division is unquestionably in many 
* Some of the adherent genera, like Tenagodus, Siliquarius, Vermetus and others, are necessarily, from their 
mode of living, Hermaphrodites. 
+ Characteristics and descriptions of so-called new species of shells are published almost daily, but occa- 
sionally this is done in such a way, that it needs no serious apology from any conchologist, for having been 
unable to decipher the true character of some of the new species. Mere outlines of casts of usually richly orna- 
mented shells certainly can be only of very limited local value. The publication merely of specific names for 
sake of priority is never justifiable. 
E 
