96 CLASS III. GASTEROPODA. ORDER VI. PULMOBRANCHIATA. 
We do not say that a complete boundary line can be marked out be- 
tween the land and freshwater mollusks, for Nature presents us with in- 
termediate modifications of habit, as well as of form or growth; many, 
for instance, are amphibious, living on the sea-shore, like the Truncatelle 
among the Cyclostomacea, or on the banks of ponds and ditches, like the 
Succinee among the Colimacea ; others again, like the Ampullarie among 
the Peristomata, located in marshy places where the water is occasionally 
dried up, are found to possess, by a liberal contrivance of Nature, a double 
system of respiration—an air-breathing system by means of a pulmonary 
cavity, and a water-breathing system by means of branchiz ;—what we 
assert is this,—that where the difference of habit is as clearly defined as 
between the Cyclostomata and the Turbines, a distinctive limit ought to be 
observed in the classification, or, in fact, wherever it may be done with 
accuracy, and without injury to the system. Variation of habit in the mol- 
lusk involves a corresponding variation in the nature and composition of 
the shell ; the difference between the influence of air and that of water, 
and the difference of food, produce a corresponding change in the compo- 
sition of the calcareous mucus which is exuded for its formation. Land 
shells are light, and never pearly or enamelled ; sea shells, on the con- 
trary, are for the most part heavy, and generally pearly or enamelled ; in 
fact, the appearance of a shell always indicates, to a certain extent, whether 
its animal inhabitant is terrestrial or marine. 
We would refer, then, to the present genus all the round-mouthed oper- 
culated species that are really terrestrial, including those varieties which 
have been separated under the title of Pterocyclos, because of the outer 
lip being furnished with an arched wing adhering to the penultimate 
whorl (Benson, Asiatic Journ., Jan. 1832), as well as those which have 
been distinguished by the appellation of Megalomastoma, on account of a 
ridge or groove in the front of the mouth, near the pillar. 
The shell of Cyclostoma may be described as being turbinated, and 
varying considerably in the arrangement of the whorls ; it is for the most 
part globose, but often turriculated, trochiform, or discoidal ; the whorls 
are round, and in their volution generally form a large umbilicus ; the 
aperture is also round, with the margins generally connected ; the lip 
