THE SNAILS, OR HELICIDA. 333 
degrees rubbed tolerably smooth in some places and encrusted in others with corallines, 
calcareous matter, and the shelly coatings of various marine zoophytes. Sometimes the 
sea-weeds find a lodgment on the shell, as is often the case with other comparatively 
stationary mollusks, such as the common limpet; and in that case the alge not only find 
a home, but conceal their protector by their waving fronds. 
The accompanying illustration represents the MARBLED CuIToN, a rather prettily colored 
shell, its exterior being rusty-red mixed with brown and yellow, and edged with brown. The 
SHort-spineD Curtron is covered 
with short spines. Its color is 
sooty-black, but this dull uniform- 
ity of a sombre hue is more than 
redeemed by the beautiful and 
minute pencilling with which its 
surface is engraved. The BANDED 
CuHITON, or CHITONELLA, has been 
removed by modern naturalists into 
a separate genus, on account ot 
the formation of the armor. The 
plates do not cover the entire sur- 
face as in the preceding genus, as 
only a portion is seen above the 
mantle. The defence is, however, 
nearly as perfect as in the previ- MARBLED CHITON. | Chiton marmoreus. 
ous genus, as the projections ap- 
proach each other beneath the surface of the mantle, and would act as effectually in shielding 
the internal organs as if the plates had met on the surface. These creatures are generally 
found in the clefts of canal rocks. 
The animal is more active than the limpet, but does not appear to be very locomotive 
in its habits. Its broad creeping disc adheres very strongly to the rocks, and holds the animal 
so firmly that, if it should happen to have taken up its abode within a crevice, to extract it 
without tools would be an impracticable task. Like the dentalium, this creature possesses 
neither eyes nor tentacles. The figures in our illustration are of natural size. 

INOPERCULATE AND OPERCULATE GASTEROPODS. 
Passine from the sea to the land, we come to those gasteropods which breathe atmospheric 
air, and are furnished with respiratory organs suited to the lower element in which they live. 
These creatures fall naturally into two large sections, the one being destitute of an operculum 
and the other possessing that remarkable appendage. They are respectively called inoper- 
culate and operculate gasteropods, and it is with the former that we have now to deal. The 
inoperculate are generally furnished with large shells; but in some, such as the slugs, the 
shell is either very small or wholly absent. The shell of these animals, when present, is not 
nearly so hard and porcelain-like as that of the sea-snails, and contains a much larger propor- 
tionate amount of animal matter. It is worthy of notice, that in order to prevent the waste of 
moisture in those species which live on land, and the entrance of water in those which inhabit 
the ponds and rivers, the respiratory passage is small, and closed with a kind of valve. 
This group embraces the largest number of species of mollusks, including snails, slugs, 
whilks, limpets, couriés, etc. The head is well developed, and one or two pairs of tentacles 
are present. The sexes are usually separate. 
THE first family is that of the Snails, or Helicidee, containing a vast number of species. 
Most of the Snails have a shell large enough to permit the animal to withdraw itself wholly 
into the protecting domicile. During the time when they are active these creatures require no 
