60 
NATURAL HISTORY. 
[east. ZOOL. 
Natica and Nacca , which have a moderate mouth, have a large oper¬ 
culum ; and Cryptostoma , which has a very large mouth, has a very 
small operculum. The operculum of Nacca is covered with a solid and 
shelly external coat, the outer surface of which is marked in a very 
different manner in the various species. The operculum of Natica is 
simply horny; Cepatia differ in the axis being covered with a large 
callosity. Mammilla chiefly differs from the latter in the axis of the 
shell being covered with a large callosity, and Naticariq, in having a 
thin oblong shell with a large oblong mouth and a thin inner lip. Cer- 
nina is imperforated with a large mouth, and the inner lip callous. This 
genus and Naticaria form the passage to Cryptostoma. The genus 
Stylina, which lives imbedded in starfish and other radiated animals, 
appears most nearly allied to this family, for what has been called the 
enlarged mantle appears to be the foot; they have a thin oblong 
transparent shell. The genus JRadula is referred here provisionally 
until its animal is known; the shell is solid and Nerite-like, with a 
rounded inner lip, having a deep notch in its centre; the throat is some¬ 
times striated. 
The animals of the following families have usually a moderately sized 
foot. 
The family of Periwinkles ( Littorinid^e, Case 20) have a free oval, 
spiral, or subspiral horny operculum; the shells generally have a roundish 
mouth, and are not pearly within ; some have a simple round mouth, 
with a concave inner lip, as the Littorina and Assiminia. Lacuna, 
differs in the pillar lip being flattened, and the axis pierced. The 
Medoria are like the Lacuna , but more solid, and covered with a 
rough periostracum. Nioma has a white spirally striated shell with a 
deeply perforated axis. The Pagodus has a top-like shelllike the Trochi , 
with which they have generally been confounded, but they have an ovate 
mouth and operculum, and the shell is not pearly. The Moduli only 
differ from them in the shell being more depressed and the inner lip 
having a distinct notch forming a tooth ; they have been confounded 
with the Monodonta. Scalaria is remarkable for having a white shell, 
marked with numerous varices, and the whorls are often only united to 
one another by the projections of the varices, thus exhibiting w hat is 
the case with all shells, that they are only formed of a coiled up, gra¬ 
dually enlarging tube. They are said to be carnivorous, like the Murices. 
Cyclotrema only differs from Scalaria in being depressed and largely 
umbilicated; and Clathrus in the whorls being closely united, as in most 
other shells. 
Bisella is like Littorina, but the shell is top-shaped, the whorls keeled, 
and the mouth rather square. 
The Solarium, or Stair-case Shells, are equally top-shaped 
and opaque, but the whorls are squarish, keeled on both sides, and 
so placed as to leave a wide conical cavity in the axis, which has been 
compared to a well staircase, hence their name. The operculum 
is ovate horny. Torinia differs in having a nearly orbicular operculum, 
which is very convex, and marked with a spiral ridge looking like a pa¬ 
goda. The Turriteltce are turreted, and marine ; the mouth of the 
shell is squarish, and the operculum orbicular, many-whorled. The 
Haustators chiefly differ in the outer lip being marked with a deep notch 
leaving a groove on the whorls. The Zaria has an ovate mouth, rather 
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