64 
NATURAL HISTORY. 
[EAST. ZOOL. 
probably a modification of the plate usually found between the whorls 
of the body. The animals of this genus have the faculty of secreting 
a cup-shaped shelly plate to support their foot, like the Hipponyces, 
but it is distinguished from the plate of the latter by being furnished 
with an oval central scar. 
The family of Carrier Shells (Phoridhs, Case 21) must be placed 
liere provisionally until their animal is better known, although it has a 
horny subangular operculum like the Zoophagous Ctenobranchiate 
JVlollusca. 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 to the outer surface 
of their shell, as it enlarges in size, stones, fragments of other shells, 
coral, and other marine substances, from whence they have been called, 
respectively, “the Conchologist ” and “the Mineralogist.” Some 
species have this habit only in an early stage, others retain it during the 
whole period of their life. The Onusti have regular shells and a large 
conical concavity on the axis, and the Phori are imperforated and gene¬ 
rally irregularly formed. 
The Heterobranchiata differ from the Ctenobranchiata in having 
variously formed gills, or respiring by means of lungs. 
The next order of Gasteropodous Mollusca are called Pleuro- 
BRANCHIata, ( 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 
within its substance. They have no operculum, and usually swim about, 
the ocean, aided by the membranaceous appendages on the side of the 
foot. 
In some, the gills are on the side of the back, and covered by the 
mantle, as the families Bullidce and Aplysiadce. 
The family of Bubble Shells (Bullidce, Case 21) have the head or 
front part of the animals without any distinct tentacula, the eyes being 
placed in a flat shield, as in the genera Bulla, Bullcea, Acera, and 
Gasteroptera; the latter has no shell, and the sides of its head are di¬ 
lated into large wings, by means of which it swims about in every di¬ 
rection. The BuU(E(E are very voracious, and prey on shell-fish, for 
which purpose they are furnished with a gizzard covered with three 
shelly plates, by means of which they can crack the shells after having 
swallowed them whole. The Bullince have the edge of the frontal disk 
produced into lobes. 
The family of Sea Hares, ( Aplys.adje, Case 21,) so called from the 
form they assume when sitting on the rocks, have an elongated head 
and distinct tentacula ; as the genera Aplysia, JDolabella , and Notar - 
ckus ; the latter has no shell, and the Aplysice emit a great quantity of 
a purple fluid. The eggs are very numerous, and are deposited in long 
threads, which are often interlaced together. The shell of Dolabella is 
hard and thick, while that of Aplysia scarcely consists of more than 
animal matter, sometimes strengthened by a thin calcareous internal 
layer. 
The family of Firoles, ( Pterotracheidh;:, Case 21,) which Lamarck 
separated into an order under the name of Heteropodes, on account of 
their foot being compressed, as in the Atalants, into a rounded, erect, 
muscular fin, with only a sucker at its hinder edge, appear to be most 
