99 
have comb-like gills placed over the back of the neck. ROOM xm. 
They are called Cteno-branchiata . Nat. Hist, 
Cases 5 to 34 contain the shells of those gasteropoda 
which prey almost exclusively on dead or living animal 
matter. These mollusca occasion the round holes which 
are often found in bivalve and other shells, and which they 
perforate for the purpose of extracting the inhabitant. 
Their shells are always provided with a canal, placed 
in the front of the pillar, which is formed to protect 
the syphon of the mantles, which conducts the water 
to their gills. Their eggs have a coriaceous envelope, 
and have often been mistaken for corals. Their oper¬ 
culum is always horny, and formed of irregular concen¬ 
tric plates: this order contains several families. 
Cases 5, 6, 7, contain the family of the Strombidce, 
which are peculiar for having a sinus formed by the 
head of the animal, and placed on the side of the ca¬ 
nal, as the true Strombus , Pteroceras , Rostettciria , 
Aporrhais , and Struthiolaria. 
Cases 8 to 19. The family of the Murices ( Mnricidce) 
which have a more or less elongated, straight canal, 
and a moderate sized, flat, expanded foot. 
The outer lip of many of these shells is thickened 
externally, forming a permanent belt across the whorls, 
as in the genera Ranella and Triton , which also have 
the innerlip generally granular; and in the Mask shell, 
Persona , with the base expanded into a disk; others 
have the inner lip smooth as those of the true Murices, 
or Rock shells. Some have no varices, or only in¬ 
distinct ones, as those of the genera Pleurotoma, Conus , 
Fusus, and Pyrula , which have the pillar smooth; Tur- 
binellus, Fasciolaria , and Cancellaria, which have this 
part plaited like the Volutes ; amongst these may be 
observed a very fine specimen of the reversed Chank 
shell, highly valued by the Chinese. 
Cases 20 to 27 contain the family of the Buccina, 
( Buccinidce ) which have either an elongated and reflexed, 
or short, curved canal in front of the mouth; and include 
the genera Cassis , Dolium Harpa } Purpura , Magilus , 
h 2 Ricinula , 
