S04 CRETACEOUS GASTROPODA 
placed either posteriorly or near the middle of its edge, which is otherwise smooth 
or finely denticulated throughout its entire extent. 
4, Nerita, Adanson, 1757 (H. and A. Adams, Gen. I, p. 378; Gray’s Guide, 
p. 186, and Chenu’s Man. I, p. 383; Otostoma, d’Archiac—ewx parte ?). Adanson 
and not Linné is the author of the genus Nerita, because the former first deter- 
mined the genus with the greatest precision, so that hardly any subsequent changes 
have been required. 
The Nerite are distinguished by the great thickness of the shell, especially on 
the inner side of the outer lip, and generally by the small number of teeth on the 
inner one. Several sub-genera have been noticed by H. and A. Adams, but I do 
not think that these can be so well defined as those of the Neritime. They call 
the depressed species with a smooth inner lip Nerita. Theilostyla, Moérch, 1852,-or 
rather Dontostoma, Klein, 1753 (Ostrac. p. 16, pl. I, fig. 29)—are similar in form but 
have the inner lip granulated, and Pila, Klein,—or Peloronta, Oken,—has the inner 
lip irregularly striated, provided with few fold-like teeth, and the spire of the shell 
short and pointed. 
A comparison of a large number of different species shows, however, that these 
sub-generic groups can be used only in a very general way, even admitting that 
there are some other minor distinctions present; for there are specimens of the same 
species to be found, some having a granulated, others a smooth, and again others a 
partially striated inner lip. 
Vise. d’Archiac proposed in 1859 (Bull. Soe. Géol. Fran., XVI, p. 871) the 
name Otostoma for a number of fossil species, which do not appear to be essentially 
different from Nerita. I have already in my ‘ Revision der Gosau-Gastropoden’, p. 47 
(Sitzb. Akad., Wien, 1865, Vol. LIT,) stated, that the species referred by d’Archiac 
to Otostoma are probably nothing more than incomplete Nerit@. Looking now at 
the state of preservation of several specimens of our Nerita divaricata and Carolina, 
T cannot but confirm my previous suppositions. I may,*however, take this oppor- 
tunity to enter a little more fully into this subject. 
Leidholt, G. Rose, and others have shown some years ago, that those shells 
which consist of arragonite very often disappear in a fossil state, leaving nothing but 
casts, while those consisting of calcite are preserved. Sorby confirms (Brit. Ass. 
Report 1862, pt. IT, p. 95) these statements by additional and new observations. 
He says that the alteration of arragonite shells ‘appears to depend on the fact of 
the particles of arragonite being in a state of unstable equilibrium. When 
prepared artificially, it has a great tendency to pass into calcite; and if this change 
took place in shells, their organic structure would be very apt to be destroyed, 
though the shell might remain as a crystalline mass of calcite. If, however, the 
circumstances of the case were such, that the calcite, formed at the expense of the 
arragonite of the shells, had a greater tendency to crystallize elsewhere rather than 
in situ, they would be removed and leave more or less perfect casts. On the 
contrary, calcite having no such tendency to change, shells composed of it might, 
under similar conditions, remain nearly in their original state,’ 
