86 
NATURAL HISTORY. [UPPER FLOOR. 
deposited by an expansion of the mantle, similar to what 
takes place in those animals. 
The next order of Ctenobranckous Gasteropodes live 
chiefly on vegetable food, and being destitute of any dis¬ 
tinct syphon for the passage of water to the branchial 
cavity, have no canal in front of the mouth of the shell: 
their eggs are membranaceous, and often deposited on 
the surface of other shells ; but many of the animals are 
viviparous. 
Case 35 contains the family of the Naticce ([Naticidce ), 
which have a very large foot expanded in front, and a 
spiral operculum; as the genera Natica , Nacca, and 
Cryptostoma , the former having a small mouth and large 
operculum, the latter a very large mouth and very smal 
operculum. 
Cases 36 to 38. The family of Periwinkles ( Littori- 
nidoe) have a roundish entire mouth, a concave inner lip, 
and a free oval, spiral operculum ; as the genera Littorina , 
Truneatella , and Valvata . 
The Melanies differ from the above chiefly by the end of 
the mouth being more or less produced into a slight canal. 
Their shells are generally turrited ,* as the genera Melania , 
Turritella , Rissoa, Scalaria, Pyramidella , Cerithiurn, 
Melanopsis , and Pyrena . Some of the latter have a canal 
like the Buccina; but their structure and mode of life, 
which is passed in fresh water, shew that they belong to 
this family. 
Cases *39 and 40. The family of Ampullar ice differ 
from all the foregoing in the operculum being annular ; 
they live in fresh water, and are covered with a thick 
periostracum. Some, as the genera Ampullarm and Cera - 
todes, have long tentacula, a forked forehead, and are 
oviparous; while the others have short tentacula, as Palu- 
dina and Bitliynia , the first of which has a horny opercu¬ 
lum, and is viviparous; the other is oviparous, and has a 
shelly operculum. 
The family of Nerites ( Neritidce ). Their shells are 
semi-ovate, with a small semi-circular mouth furnished 
with a sharp transverse inner lip ; as the genera Nerita, 
Neritina , and Navicellus, the former has a shelly opercu¬ 
lum grooved on the edge, and the two latter, a thin one 
with a flexible margin. 
