95 
EAST. ZOOL. GAL.] NATURAL HISTORY. 
their shape and manner of life. The shells of this family 
vary considerably in shape; indeed, so much so that it is diffi¬ 
cult to believe how they could be formed by the same kind 
of animals, yet, when a large series of them are compared, 
they so gradually pass one into the other that it is not easy 
to separate them even into genera. In Crepidula and 
Calyptrcea the apex is spiral; in some of the latter genus 
the axis is perforated. In some species, otherwise allied to 
Calyptrcea , the spire is so reduced in length, and the la¬ 
mina which separates the whorls of the body is so rudi¬ 
mentary, that nothing but the part which surrounds the 
much enlarged perforation of the axis is left; these modi¬ 
fications, when fully developed, have been called the Cup 
and Saucer Limpet ( Dispotea ). The Cremoria , on the other 
hand, have only a folded plate on the apex of the cavity to 
support the back of the body, which is probably a modifica¬ 
tion of the plate usually found between the whorls of the 
body. These animals also have the faculty of secreting a 
cup-shaped shelly plate to support their foot, like the Hip - 
ponyces , but it is distinguished from the plate of the latter 
by being furnished with an oval central scar. 
The family of Carrier Shells ( Pkoridce , Case 21) must be 
placed here provisionally until their animal is better 
known, although it has a horny subangular operculum 
like the Zoophagous Ctenobranchiate Mollusca. The foot 
is moderate, and the eyes are at the base of the subulate 
tentacles. Their shell is conical, much like a Calyptrcea in 
shape, but they have the peculiarity of attaching, as the 
shell is enlarged in size, stones, fragments of other shells, 
coral, and other marine substances to its outer surface, from 
whence they have been called, respectively, “ the Con- 
chologist ” and “ the Mineralogist/' Some species have 
this habit only in an early stage, others retain it during 
the whole period of their life. 
The Heterobranchia, differ from the Ctenobran- 
chiata in having variously formed gills, or respiring by 
means of lungs. 
The next order of Gasteropodous Mollusca are called 
Pleurobranchiata, (Case 21,) from their gills, which 
are composed of plates, being placed on the right side 
of the back, and covered with a thin mantle, which is 
sometimes protected by a small shell, more or less sunk 
