PHYSA.- 
•MOLLU SCA. Pteropoda. 
356 
Genus Physa. — The Bubble Snails have a sinistra] 
shell, of an ovate form, spiral, thin, and polished. The 
aperture is oval, rounded in front, the inner lip spread 
over the last whirl, and the outer lip acute. The species 
may easily be recognized by their bright polished sur- 
face, and their sinistral or left-handed whirls. The 
[di 3 ’S 8 e are chiefly found in clear running streams and 
in fountains, seldom in stagnant water. They appear 
to be very quick in their movements, and when touched 
or alarmed, thej' onl}' arrest their progress for a moment, 
and then go on again without further hesitation. When 
floating on the surface of the water, they possess the 
power of letting themselves down gradually by means 
of a glutinous thread affixed to the surface (as in the case 
of Litiopa — see p. 204) and when kept in a glass vessel, 
they have been observed frequently to poise themselves 
on their foot, and rapidly vibrate their shells. The 
Physas are not so numerous in species as the Lymncece, 
but are widely diflused, being found in Europe, North 
America, South Africa, India, and the Philippine 
Islands. This family and genus are represented on 
Plate 7, fig. 1 {Physa rivalis). 
The Genus Planokbis differs very much in ap- 
pearance from the two preceding genera. The shell 
is discoidal, instead of spiral, with a depressed spire 
and numerous whirls, which are visible on both sides. 
The mouth is crescentic, and the peristome is thin, 
sharp, incomplete, the upper margin projecting. The 
species are numerous, upwards of sixty having been 
described, and are in a great majority of instances 
natives of temperate climates, as Europe and North 
America, &c., though a nrimber are found also in India 
and China. Locomotion in these animals is slow; and 
it has been observed in the case of a common British 
species {P. conmis) that when irritated, it discharges 
a purple fluid from under the margins of the mantles, 
similar to that of the Scalaria (see p. 200), but which is 
of so fugitive a nature that no means tried yet have 
been able to preserve it. 
Thalassophila — Marine Snails. — In this group 
the animals have no distinct tentacles, or they are 
expanded so as to form a frontal disc, upon the front 
part of which are placed the eyes. Tliey are either 
marine in their habits, or they live in the vicinity of 
the sea. 
Family— SIPHONARIID^. 
The Siphonarias have their frontal disc bilobed an- 
teriorly, and the respiratory orifice covered by a large 
fleshy lobe of the mantle. The shell is conical, and 
exactly resembles a patella, except that the horse-shoe- 
shaped muscular impression is divided on the right side 
b}”^ a deep siphonal groove, which extends in the form 
of a projection somewhat bejmnd the margin. In form 
it is depressly conical, with the apex subcentral, and 
recurved posteiiorly. There is onlj' one genus, Sipho- 
naria. The species, upwards of thirty in number, are 
found adhering to rocks, like limpets, between tide 
marks, and are widely extended in their geographical 
distribution. They are most numerous within 'the 
tropics, being found in considerable abundance in India, 
the Philippine Islands, South America, the Cape of 
Good Hope, Australia, .New Zealand, and the islands of 
the Pacific. The family is represented on Plate 7, 
figs. 1, 2, 3 {Sip)honaria racUata and granclis). 
Family— AMPHIBOLID^. 
The Amphibolas have a subglobose, spiral, umbili- 
cated shell, with an oval mouth and a rather expanded 
outer lip, which is sinuated behind. The surface of the 
shell is singularly uneven, and battered as it were. 
The animals possess an operculum, which is horny and 
subspiral. 
Only one genus, Amphibola, exists. 
The Amphibolas are found living in salt marshes near 
the sea. They exist in great abundance in New Zea- 
land and Australia, living in pools of brackish water, 
and at particular seasons burying themselves in the 
sandy mud. The natives of New Zealand use them as 
articles of food. 
Class III. — PTEKOPODA (the Pteropods). 
The true position of the Pteropoda in the natural 
arrangement of molluscous animals, is still somewhat 
undefined. Some conchologists place them between 
the Brachiopods and Cephalopods, others between the 
Gasteropods and Brachiopods. Lamarck conceived 
they should be placed between the Gasteropods and 
Bivalves, and M. Blainville is of opinion that they are 
in reality only a tribe of Gasteropods, allied to the 
Bullidw. M. Souleyet supports this view of their posi- 
tion, though the structure of the animal evidently 
entitles them to rank as a distinct group or class. The 
animals resemble very much the larval or young state 
of the Sea-snails; and the shell, when present, is dis- 
tinctly univalve and analogous to the shell of the Gas- 
teropods. They are, however, much inferior to this 
class of mollusks, and we place them therefore after 
the Univalves in our arrangement. 
The animal in most of the species is inclosed in a 
thin shell, which is either conical, cylindrical, or sub- 
globular. It is symmetrical, and consists of a dorsal 
and a ventral plate united, with an anterior, transverse, 
contracted opening for the head, and lateral slits for the 
passage of certain long filiform processes of the mantle. 
The sexes are united in the same individual. 
The Pteropods are amongst the most beautiful of all 
the Mollusca, but are seldom seen alive except by those 
“ whose business is in the great deep.” In consequence 
of the great fragility of their shells, they avoid the 
shore, and are generally met with only in the open 
ocean. They are the most entirely natatory of all the 
animals of this kingdom. “ Created,” says Dr. John- 
ston, “ to occupy the high seas, they are organized in 
evident aptitude to the place assigned them, with a 
light shell, which afiects not their buoyancjq and with 
