206 PECTINIBRANCHIATA.—TROCHID A. 
often covered and disguised by irregular incrusta- 
tions of calcareous matter, deposited by the water, 
which may serve as a protection to the animals, 
causing them to take the appearance of rough 
stones or masses of earth. This species, like all 
its fellows, displays very little of its body when 
crawling. 
FAMILY TROCHIDA. 
(Top-shells.) 
An extensive group of very fine shells is in- 
cluded under the above appellation, many of which 
are of considerable size, of very regular and ele- 
gant shape, and of exquisite beauty of colour, and 
sometimes of sculpture. Our own shores possess 
many species, among which are some of the finest 
of our univalve shells, and specimens distinguished 
by all of these characteristics. Yet the finest 
species are, as usual, exotic, and tropical; for the 
great Pearly Top (Zrochus pica) of the West 
Indies, the Imperial Sun (Imperator imperialis) 
of Australia, and the Perspective Staircase (Sola- 
rium perspectivum) of the Indian Seas, belong to 
this family. 
The shell in this large group varies considerably 
in form, but is always spiral; the spire sometimes 
is drawn out to great length, at other times so 
much depressed as to be nearly flat; but it always 
forms a large portion of the shell. The aperture 
is entire, without notch or canal, as the animals 
are destitute of a siphon. 
The animal has a head terminating in a broad 
muzzle, and often ornamented with head-lobes ; 
side-lobes greatly developed, and furnished with 
