114 
NATURAL HISTORY. [EAST. ZOOL. 
with a thick periostraca, but some, as Phasianella , are 
covered with a very thin transparent one, shewing the 
brilliant colours of the shell through it. The Turbo Sar - 
maticus is peculiar for having a layer of blackish animal 
matter between the outer opaque and the inner pearly coat 
of the shell; this coat forms a dark zone between the edges 
of the two coats, just within the aperture of the shell. 
The family of Trochidce (Case ) chiefly differ from 
the former in having a more conical shell, with a square 
mouth; the operculum, which is generally formed of 
many slowly enlarging whorls, is destitute of any shelly 
coat; and the bases of the tentacles are without appendage. 
The family of Stomatellidce (Case ) are very like the 
former, but the mouth of the shell is large and expanded 
compared with the size of the spire and operculum. 
The family of Haliolidce (Case ) have even a more 
expanded mouth than the former; they have no opercu¬ 
lum, and the side of the foot of the animal is covered with 
a hard warty skin; but their most peculiar character con¬ 
sists in their having usually a groove or slit in the mantle, 
over the part where their gills are situated. In Stomatia 
the place of this slit is marked in the shell by a groove, 
while in Halioils , and the allied genera, the groove of the 
shell is pierced with a series of holes in front, which are 
gradually filled up behind as new ones are required to be 
formed by the growth of the animal, the last being occu¬ 
pied by the vent; and in Scissurella and Pleurotomaria 
there is a long slit in the shell, over the opening in the 
mantle. 
The family of the Keyhole Limpets ( Emarginulidce , Case 
) have an animal very like the Ear-shells, but the shell is 
depressed, simply conical, not pearly, and furnished either 
with a hole, placed in the front of the apex, as in Fis - 
surella, or with a notch in the front of the edge, as in 
Emarginula and Parmophorus . These holes or grooves 
afford a passage for the water to the respiratory organs, 
and for the expulsion of the faeces. In Pupillia the shell is 
surrounded by a sharp white edge; and in Lucapina the 
mantle covers the cancellated shell. 
Here must also be placed the family of the Tooth-shells, 
( [Dentaliadce , Case ,) which have been, till lately, re¬ 
garded as the tubes of worms, but are now known to be 
