File:The Quarterly journal of the Geological Society of London (13364960285).jpg

1849..
SHARPE ON THE GENUS NERIN^A.
103
only, as the perforation of the columella is not accompanied by any
other constant character, and the umbilicated and non-umbilicated
species can hardly be distinguished ; moreover N. Voltzii of Des-
longchamps has the columella solid when young and perforated in
its older stages. More than half the known species of Nerincea fall
into this division.
Fig. 1 is the section of a portion of i\^. Archimedis, D'Orb., from
near Cintra, which illustrates the internal characters of this sub-
genus.
Fig. 2.
Fig. 1.
Subgenus 2. Nerinella.
Columella solid, either simple or furnished with one fold; the
outer lip with one internal fold ; folds simple ; the mouth usually
much longer than wide ; the shell nearly cylindrical or very taper.
The species of this section are not numerous ; they have not the
characters laid down by the founders of the genus, but are obviously
closely related to it : they are usually small and very elongated.
Fig. 2 represents a section of part of iV^. Dupiniana, D'Orb., copied
from the * Paleontologie Fran9aise ' to illustrate this subgenus.
Subgenus 3. Trochalia*.
Columella umbilicated, with one fold ; mouth rhomboidal ; the
outer lip either simple, or thickened internally, or furnished with
one internal fold ; folds simple ; the shell usually short and co-
nical.
These species differ widely from the original type of the genus,
and approach the Trochi ; they are usually of large size, without any

• Named fi'om its resemblance to Trochus.
  

TpoxaXia, a water-wheel.