101 
Case 35 contains the family of the Naticce (.lAaticidce ), 
which have a very large foot expanded in front, and a 
spiral operculum; as the genera Natica , Nacca, and 
Cryptostoma 5 the former having a small mouth and 
large operculum, the latter a very large mouth and 
very small operculum. 
Cases 36 to 38. The family of Periwinkles, ( Litton - 
nid(E ,) have a roundish entire mouth, a concave inner 
lip, and a free oval, spiral operculum ; as the genera 
Littorina , Truncatella , and Valvata. 
The Melanice 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 , Ce- 
rithium , 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 Ampullarice , 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 Ampullaria and Ce- 
ratodes , have long tentacula, a forked forehead, and are 
oviparous ; while the others have short tentacula, as 
Paludina and Bithynia , the first of which has a horny 
operculum, and is viviparous; the other is oviparous, 
and has a shelly operculum. 
The family of Nerites ( Neritidee ). 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 
operculum, grooved on the edge, and the two latter, 
a thin one with a flexible margin. 
Case 41 contains the shells of those Mollusca of this 
order, which are peculiar for having their branchiae 
formed of very long filaments, as the family of Worm 
Shells, (Vermetidce,) which are often attached by the outer 
surface to marine bodies; being thus fixed, the foot is not 
furnished with a distinct disc for walking, but its end is 
expanded, 
ROOM XIII. 
Nat. Hist. 
