OF SOUTHERN INDIA. 5 
3.—The proportion of the height of the last, and, occasionally even of the 
penultimate whorl, to the height of the axis. 
It must be understood, that these and any other measurements regarding the 
actual size of shells will be only noticed, when they can be of use in the recon- 
struction of the shell, or when they are in other respects characteristic. The scale 
used will be, as before with the Cephalopoda, the millimetre. There is little use 
in quoting measurements of already figured specimens, or even actual measurements 
in general, excepting as regards the usual size of a species, or its greatest variation. 
_In the descriptions of species we shall, as usual, give first a diagnosis in Latin, 
while the rest of the description will generally be more explanatory, than a mere 
translation of the diagnosis. Observations on similar species already known, and 
comparisons with these will be carefully noted, because these are in many cases 
of far greater importance, than any imperative characteristic of a new species. 
Regarding known species, we shall continue to notice largely all facts of identity 
and affinity, and to illustrate them amply with the specimens at our disposal. 
Special geological interest always attaches to the occurrence of these known species. 
The same system will be retained in referring to the different genera and families, 
and to their geological distribution in time and space, as well as to their concholo- 
gical relations. 
It is not necessary at present to make it an object to enumerate all known 
Cretaceous fossil species of the genera described, as this werk has only just been 
completed by Prof. Pictet (Mat. p. 1. Pal. Suisse, 3rd Ser. 1864.). If, however, 
any particular additions or considerable alterations have been made since the date 
of Pictet’s publications, we shall not omit to attend to this subject. In 1861 
(Proc. Am. Phil. Soc. Vol. VIII.) W. Gabb published a catalogue of what he 
then supposed to be all known Cretaceous Mollusca. The number of fossils, however, 
omitted from this catalogue is very considerable, and a little want of care in the 
critical examination greatly reduces the value, which such a publication, if properly 
executed, would so highly deserve. 
