OF SOUTHERN INDIA. 277 
These studies on the changes of organisation, as produced by different modes 
of life, characterize our progress in natural science of later years, a step in progress 
towards the solution of that still mysterious question, the development of organic 
life; they are, however, the only means through which we can ever hope to arrive 
at a system deserving the name natwral. From the instructive tablets, referring 
to the development of specific distinction in the genera Rissoma and Rissoa, as 
given in the Monographs of Schwartz von Mohrenstern, it is clearly visible how 
much that author appreciates the importance of this course of study, and it is only 
to be hoped, that he will extend his already successful investigations to all the forms, 
which appear to constitute this natural family. 
Tn our cretaceous deposits of Sth. India only three genera have been found 
represented, and they are at the same time those, to which,—speaking generally,— 
every cretaceous species of Rzssoz., hitherto discovered, belong. 
LVIII. RISSOA, Fréminville, 1814. 
Char. Riss. testa oblonga, turrita, crassa ; anfractibus sepissime transversaliter 
costulatis; apertura ovata, mtegra, marginibus junctis; labro paululum expanso, 
extus varice imcrassato, intus levi; columella solida, in medio labii truncate termi- 
nanti seu torta. (Vide Denksch. Akad., Wien, 1864, Vol. XXIII, pt. II, p. 4,— 
with slight alterations). 
Schwartz von Mohrenstern is of opinion, that the genus Rissoa, as restricted 
within the above characters, has no representatives below the tertiary epoch, but 
T cannot help thinking that he is misled in this statement, unless we can admit 
that different genera ought to be framed for different formations. When we base a 
generic distinction upon great differences existing between the animals of two very 
similarly formed shells, we are entitled to do so, because some distinctions will 
also generally be afterwards traced in the shells. Where, however, these latter are 
the only materials accessible to our examination, we cannot go beyond the charac- 
teristics, which we regard as generically common to a certain number of shells. I 
do not say anything about the jurassic species, although I am convinced that 
several of them are true Rissoe. As to the existence of several cretaceous species 
of Rissoa, there cannot be, however, any serious objections made. I will mention, 
for instance, the species known as Rissoa Dupiniana, D’Orb. (Pal. france. crét. IT, 
pl. 155, figs. 8-10,) from the Gault of France, and another as Rissoa affinis, Sow. sp., 
from the Gosau-deposits (Sitzb. Akad., Wien, 1865, Vol. LII, Stoliczka’s Revision, 
etc., p. 19). I add to these, two new species from South India &. Oldhamiana and 
R. tropica. 3 
As very doubtful species, which most probably have to be excluded from 
this family, may be mentioned R. incrassata and R. Sandbergeri, Miller (Petreef. 
Aachner Kreidef., 1851, pt. I, pp. 76 and 77), both of which would rather belong 
to the vassivz of the Bucciniox, although this could only be proved by the 
oY 
