PULMONATA. 59 
with the modifications of the shell. In the extensive family of the Helicide, most 
especially, has this occurred, and many of the genera ‘thus formed are consequently 
either wholly rejected, or received provisionally until it is ascertained by further 
examination whether or not there is anything except merely artificial characters to 
support them. 
The Gasteropods forming this order are all phytophagous. They are very widely 
spread, being found in almost all parts of the earth, but they principally abound in 
warm or tropical climates, where the largest species occur. They are, for the most 
part, inhabitants of land, but many live in water, coming to the surface for respiration ; 
of those which live in water, the greater number inhabit ponds, running streams, or 
stagnant waters, but some few are marine animals, frequenting the shallow sea near 
the shores, or salt-water marshes. 
Two distinct forms of the opening by which the communication between the 
pulmonary sac and the external air is kept up, are presented; and as each appears to 
be accompanied by corresponding peculiarities of organisation, Mr. Gray has availed 
himself of them for dividing the order into the two sub-orders, Adelopneumona and 
Phaneropneumona.* In the first division the communication is through a lateral orifice 
formed by the edge of the mantle, which, except at that part, is united along the left 
side of the animal; in the second division the edge of the mantle is free or detached 
along the nape, leaving the pulmonary cavity open. ‘The animals comprised in the 
first division are all hermaphrodite, and without an operculum ; while, on the contrary, 
those which form the second division are unisexual and operculated. The genera 
constituting this order had previously been divided, according to their habitats, into 
terrestrial and aquatic (¢erricola and aquatica of Dr. Fleming); a mode of distribution 
which brought together animals presenting important zoological differences. This 
principle of subdivision may, however, be used with convenience in the Adelopneumona, 
and Mr. Gray has, in fact, divided that sub-order into the three sections, Geophila, 
Limnophila, and Thalassophila, the last two representing the aquatica of Fleming.t 
Mr. Webster many years ago, (‘ Geol. Trans.,’ vol. ii,) noticed the occurrence in the 
Purbeck beds of fossils resembling fresh-water shells, and in the Museum of Practical 
Geology is a series of fossils from that formation, comprising Limneea and Planorbis. 
Fossil remains, referred to the genus Avricula, have also been found, according to 
M. Nyst, in the chalk formation in the department of the Aube in France. In the 
* Etym., AdnAos, (hidden, concealed,) and gavepos, (open, exposed,) prefixed to rvevpwy, (the lungs.) 
These sub-orders correspond with the divisions inoperculata and operculata, proposed by Férussac, and 
adopted by Dr. Turton, M. Rang, and others, but as the names used by Mr. Gray express modifications of 
the respiratory apparatus, which forms the character of the present order, I have adopted them, although 
the words operculata and inoperculata are preferable from their simplicity. 
+ Etym., yea (land), Acuxn (a pool or marsh), and @adacon (the sea), respectively prefixed to guXos 
(loving). 
