ROOM XIII.] NATUKAL HISTORY. 8J 
cells are half the length of a whorl, so that each new cell 
changes the situation of the mouth from one to the other 
end of the shell, as in the Miliolce : and in others the cells 
are divided into numerous longitudinal tubes, as in Alveo- 
Lina and Fabularia, 
In Cases 5 to 60 are arranged the shells of the Gaste- 
ropodous Mollusca—which walk on a broad, flat, ventral 
disk. All these animals have a single spiral shell, except 
those belonging to the genus Chiton , which have a series 
of valves down the back. The Gasteropoda are divided 
into orders according to the form of their respiratory 
organs. 
The greater number of those furnished with shells have 
comb-like gills placed over the back of the neck. They 
are called Cleno-branchiata . 
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 operculum is always 
horny, and formed of irregular concentric 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 canal, as the 
true Strambus , Pteroceras, Rostellaria , Aporrhais , and 
Strut hiolaria. 
Cases 8 to 19. The family of the Murices, ( Muricidce ,) 
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 ex¬ 
ternally, forming a permanent belt across the whorls, as in 
the genera Ranella and Triton , which also have the inner 
lip 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 indistinct ones, as those of 
