OF SOUTHERN INDIA. 335 
The shell of all Nzrrrrpx consists of two very distinct layers, which are 
especially well developed in most of the typical marine species of Nerita. The outer 
layer is a White milky substance with rhombohedral fracture, like calcite; the inner 
one, usually known as the inner callosity of the shell, is in recent species of 
Nerita a more or less homogeneous, transparent mass. Before the blowpipe both 
layers of the shell stand the heat equally well, and are reduced to quicklime ; 
from which it would appear, that they are both calcite. I have at least no direct 
means at present to show that the inner layer is arragonite, for only the pure 
crystalline masses of this mineral seem readily to fall to powder when exposed to 
heat. Impurer masses of arragonite do not fall to powder before the blowpipe, or at 
least not so readily. 
It is certain that the inner layer of the shell of the MNerite is even to the 
naked eye very different from the outer one, and whether it be arragonite or 
calcite, the statement of Mr. Sorby, that the particles are in a state of a certain 
unstable equilibrium, appears to be thoroughly applicable to our case. In several 
of our specimens of Nerita divaricata the inner or callose layer of the shell is 
changed into an aggregate of well developed crystals of calcite; (vide Pl. XXVIII, 
Fig. 5); in other specimens it has partially or perfectly disappeared. In both 
these cases the outer layer remains perfectly unchanged. This is therefore fully 
in accordance with Mr. Sorby’s statement, which I have quoted above, namely, 
that the shell may either remain ‘as a@ crystalline mass of calcite’, or, under 
certain circumstances, be ‘7emoved’ altogether. This removal of the inner layer of 
the shell produces, however, a remarkable change in the appearance of the shell, 
inasmuch as it loses all its distinguishing generic characters. <A thick and massive 
shell provided with a large flattened, dentated inner lip and a narrow aperture 
becomes thin, without a trace of any thickening and witha very large roundish 
aperture like a Velutina. A glance at Binkhorst’s figure 1 b, of Nerita rugosa 
(Monog. Gast. et Ceph. de la craie de Limbg., 1861, pl. 5a,) shows very clearly the 
thickness of the callous layer which has been dissolved away, and at the same time 
the remarkable difference in the aspect of the shell (see also ibidem, pl. 3, fig. 1dc). 
T may remark that the removal of the callous layer appears to have taken place in 
some instances by such a gradual and slow process that its place has been taken by 
the surrounding rock, without producing the slightest change in the external form 
of the shell. It is in such cases extremely difficult to detect, in these apparently 
well preserved shells, species of Nerita. 
The Ner. rugosa, which was generally described as a Natiea,—until Mr. Bink- 
horst discovered the internal casts of the shell,—was one of the three cretaceous 
species upon which Vise. d’Archiac based his genus Otostoma. Even in Techihat- 
cheff’s ‘Asie Mineure, Paléontologie,’ 1866, p. 89, the same species is retained 
under the generic denomination of Otostoma, though with a query, but without 
making reference to Mr. Binkhorst’s well known discovery! With regard to the 
Nerita rugosa from Maestricht, Ner. nodosa (Natica id. olim) from Bohemia and 
Saxony, Ver. Zekeliana and Ner. Hérnesiana (Natica rugosa and Hornesana of Zekeli) 
