MOLLUSCA OF INDIA. 157 
The examination of many different species of Macrochlamys and 
those of allied genera begins to afford sufficient material on which to 
base their classification ; and I am now convinced and hope to show 
that a system commencing with Helicarion, as if that were a primor- 
dial form, cannot be adopted. I think I have given sufficient examples 
of the modification of the shell-lobes of the mantle in these genera 
and subgenera to show clearly that their gradual growth has been 
in the direction from genera without such appendages to those that 
possess them. Climate has, no doubt, had a great influence in 
producing this change, certainly facilitating the growth of it. The 
highly-heated damp atmosphere of the Khasi Hills, particularly the 
southern side, where for hours the air is in a state of complete satu- 
ration, tends to the enlargement of such appendages, until we reach 
forms like Austenia bensoni, planospira, &c., and with still greater 
extension and spread of the same part of the mantle in species like 
Girasia hookeri &c., where the junction of the two originally dis- 
connected lobes is so admirably shown by the distinct cicatrice along 
the line of junction (P. Z. 8. 1880, pl. xxiv. figs. 1, 2, and pl. xxvii. 
fig. 1), indicating that even in the young the lobes are not quite 
united, but do so as development proceeds, an interesting case of the 
tendency of homologous parts to cohere, one of the many interesting 
facts put forward by Darwin. The interesting result is yet to be 
noted, that as the animal thus gains an extra protection to the 
most vital organs of the body within the spire of the shell, this latter 
structure becomes less and less developed until we find only a mere 
thin horny or membranaceous shell; for instance, in forms such as 
Girasia burtii, where the shell-lobes unite completely, the apex of 
the spire is reduced to a mere nucleus, of rounded form, at the end 
of a narrow strip of thin membrane, which readily breaks off from it. 
This cannot be considered a retrogression or deterioration in develop- 
ment; from a pure conchological point of view it may be, but in 
fact it is a great stride in advance. 
This slug-like form which, having got rid of a structure superflu- 
ous to its existence and a great tax on its powers to construct, re- 
mains as perfectly developed in all its organs as the most highly 
specialized mollusks with solid shells, and is able to hold its own 
against its numerous enemies ; in fact, its form is greatly in favour 
of its prolonged existence, seeing that it is better able to conceal 
itself deeper among the crevices of the rocks, under stones, and in 
the cracks of fallen trees, or underneath the bark, situations inac- 
cessible to most of the shell-bearing species, certainly to all those 
of equal size. I do not think it is possible to find a better instance 
of gradual development of species, 2. ¢. with so many existing links 
presented to us by these interesting forms of the Zonitide. It is 
not difficult to imagine a similar kind of evolution from some heli- 
coid ancestor developing a form like our common Arion or Limax. 
Development has in this case proceeded further, for in these two 
genera the body-cavity extends to the extremity of the foot, and 
is occupied by the coils of the intestine, the liver-lobes, &c. Now 
