62 
NATURAL HISTORY. 
[east. ZOOL. 
tomia has a single plait on the pillar, and the operculum has a flap, but 
the animal is unknown. The genus Nerinea , which is established on a 
number of turreted fossil shells, appears allied to this family; it is pecu¬ 
liar for the outer side of the cavity as well as the pillar being furnished 
with spiral ridges ; the outer sides of the cavity of Pyramidella , and 
some Cerithia , only have the toothed cross ridges placed at certain dis¬ 
tances, and never continued ones. 
The family of Turned Shells (Tornatellide, Case 21) also have 
plaits on the pillar, and a horny operculum with a flap; but the animal 
has no true tentacles, only an expanded disk on the front of the body, 
somewhat like the disk of the Bullidce. The genus Tornatella have 
thin shells with oblique plaits on the pillar; the Solidulce have very 
heavy shells with a callous inner lip and two plaits. The Cinulia are 
like the Tornatella with two plaits, and the outer lip is thickened ex¬ 
ternally. They are all fossils, and Monotygma has only a single plait. 
2. The animals of the families which follow have their gills formed 
of long filaments, and often exserted when the animal is expanded; 
their shells are very variable and anomalous in form, and often have a 
very large aperture. 
The family of Valve Shells ( Valvatide, Case 21) are small fluviatile 
Mollusca, found in rivulets, with an orbicular many-whorled spiral 
operculum; they are remarkable for having their gills, which are 
formed of long spiral plates, protruded on the outer side of their base 
beyond the shell when they walk. The shells are regular spiral, with a 
round simple mouth, and covered with a pale olive periostraca. 
The family of Worm Shells ( Vermetidce , Case 21) are peculiar for 
having an irregularly tubular shell, which is generally attached, by its 
outer surface, to shells and other marine bodies. The body is elongate, 
and living thus fixed, the foot is not furnished with a distinct disk for 
walking; its two ends are folded together, and its hinder end is pro¬ 
duced into a flat orbicular disk, as large as the mouth of the shell, which 
is generally protected by an orbicular horny operculum. The operculum 
is very variable in form; it is sometimes concave, with a central scar, at 
others it is formed of many whorls with a thin raised outer edge, and in 
Siliquaria , after the animal has arrived at its full size, it continues to 
form new whorls to the horny operculum, which are all of the size 
of the mouth of the shell. This latter genus is also peculiar for the 
mantle having a slit near the edge of the gill, as in the Haliotides, and 
its tubular shell is also furnished with a similar slit. The genus Spiro - 
glyphus , instead of a tube, forms a groove on the surface of other shells, 
which it covers over with shelly matter and converts into a tubular case 
for its body; in their young state these animals assume a regular spiral 
form, but after a time they often take another direction. It is very 
difficult to distinguish the shell of the Vermeti from the temporary 
shelly case formed by some kind of worms, as Serpula ; some species 
appear to be destitute of any operculum. The Vermetus has a con¬ 
cave orbicular operculum with a rugose central scar. The Bivince have 
an orbicular spiral operculum with an oblong lateral scar, like the 
Trochi. The Vermilia has the mouth of the tube surrounded by three 
spines, and the Hatina has no operculum. 
The family of Vanicoroide appears to unite the Vermetidce to the 
next family; they have a cancellated white shell, somewhat like an um- 
