2 CRETACEOUS GASTROPODA 
As is generally the case, the Prosobranchia are, in the rocks referred to, by 
much the most numerous. It would be, no doubt, a more natural course to 
open our descriptions with the lowest group—Scaphopoda—but following the ar- 
rangement already adopted in the previous descriptions of a portion of the Mollusca 
from the same rocks (see Paleeontologia Indica, Cretaceous Cephalopoda of Southern 
India) we shall begin with the highest order, the Pulmonata. Generic charac- 
teristics will be given in greater detail where necessary, and especially, when a 
genus is introduced for the first time into the literature of cretaceous Iollusca. 
Cases will sometimes occur, when a more general,—usually an older denomination 
of a genus,—is retained as the principal name, while a second is given in a parenthesis. 
The latter usually refers to a name used for the genus in a more restricted, and 
generally a newer or more recent, sense. We would simply repeat here, that we 
prefer leaving room for further correction by exposing the insufficiency of the 
materials at our command, rather than by using terms of fixed meaning to attempt 
to give an apparent, but possibly fallacious, completeness to the descriptions. Even 
in some well preserved fossil shells, there still remains for the most part so much 
that is doubtful, and so much that must be mere supposition, that we must be 
careful to avoid any hasty descriptions of imperfect specimens, which would fre- 
quently admit of more explanations than one. 
The geological terms to be used in reference to the cretaceous rocks of Southern 
India, will be the same* as those already used in the descriptions of the Cephalo- 
poda. Four groups of beds are distinguished; the Arrialoor, Trichinopoly and 
Ootatoor groups in the Trichinopoly district, and the Valudayur group from the 
neighbourhood of Pondicherry. The last (the Valudayur) may be taken as nearly 
equivalent to the lowest division (the Ootatoor) of the Trichinopoly series, although 
the equivalence seems to be only partial. Many of the fossils, which were examined 
from these beds, appear to indicate, that some Arrialoor beds had been classed with 
them, and it may even be possible that these younger beds constitute the greater 
portion of this group. Very strict boundaries between each of the groups were not 
defined by means of the geological or stratigraphical examination of the rocks. 
We must hope that the paleeontological enquiry, when extended over the entire 
fauna, may throw some light upon the value of these divisions, and that the dis- 
tribution of the various groups of fossils will help to explain much as regards the 
co-existence, or succession in time, of the various deposits. 
We may here anticipate the remark, that while the Ootatoor group has yielded 
the largest number and greatest variety of Cephalopoda, the Arrialoor and Trichino- 
poly groups contain to the same degree a much richer fauna of Gastropoda. At the 
close of the detailed descriptions, the results, as regards this class, will be given in 
the same way, as already attempted with the Cephalopoda. If not specially mentioned 
it will be understood, that the specimens described or figured are deposited in 
the collections of the Geological Survey of India. With regard to nomenclature 
* These will be found more fully treated of in Mr. H. F, Blanford’s report in the 4th Volume of the 
Memoirs of the Geological Survey of India. 
