PULMONATA. 59 



with the modifications of the shell. In the extensive family of the Helicidae, 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, Addopneumona 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 {terricola 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, Geop/ti/a, 

 LimnojjJiila, and Thalassophila, the last two representing the aquatica of Fleming. f 



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 Limnaea and Planorbis. 

 Fossil remains, referred to the genus Auricula, have also been found, according to 

 M. Nyst, in the chalk formation in the department of the Aube in France. In the 



* Etym., ASijXos, (hidden, concealed,) and ipuvepos, (open, exposed,) prefixed to irvtvfiuv, (the lungs.) 

 These sub-orders correspond with the divisions inoperculata and operculata, proposed by Ferussac, 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. 



■f Etym., yea (land), Xi/j»»j (a pool or marsh), and QaKaaar) (the sea), respectively prefixed to <pi\os 

 (loving). 



