1824.] Mr. Gray on the Pulmonobranchous Mollusca. 107 



Article VI. 



On the Natural Arrangement of the Pulmonobranchous Mollusca. 

 By John Edward Gray, MGS. 



(To the Editors of the Annals of Philosophy .) 



GENTLEMEN, July 5, 1824. 



Being dissatisfied with the arrangement of the air-breathing 

 mollusca proposed by Ferrussac in his showy work on the Land 

 and Freshwater Shells, which he appears to have thought imper- 

 fect himself, as he has proposed a new one in one of his late 

 numbers, which I consider to be more artificial, and consequently 

 inferior to his former order, I have sent you a sketch of 

 their circular disposition, which I had considered as the proper 

 linear disposition of them before the publication of Mr. W. S. 

 Macleay's excellent views. 



The animals of this order of mollusca are at once distinguished 

 by their closed pulmonary cavity, on the parietes of which the 

 aerating vessels are reticulated. They all breathe free air; for 

 those animals which live constantly in water rise to the surface 

 to replenish their stock of that fluid so essential to life. They 

 have no operculum, and this latter character at once distin- 

 guishes the shells of this family from the Cydostomida, which 

 appear to be the connecting family between them and the Tur- 

 binidce. 



The true land animals of this group, as the slugs and snails, 

 which may be considered as the type, are characterized by their 

 tentacula always being capable of being withdrawn into them- 

 selves, as the finger of a glove, and by their eyes always being 

 placed on a pedicle capable of similar retraction ; whereas those 

 which are almost constantly found in or floating on the surface of 

 water, as the Land-ears, the Pond and Shield-snails, are fur- 

 nished with contractile tentacula and have their eyes usually 

 sessile at the base of them. The groups of this order may, there- 

 fore, be thus characterized : — 



1. Tentaculis retract 'ilibus, oculis pedicillatis. Terrestres. 



Mollusca gasteropoda LlMACIpJE. 



Mollusca trachelipoda, pallii marginibus in-~lrr 

 crassatis J 



2. Tentaculis contractilibus. Aquatica;. 



achelopoda, pallii marg 

 crassatis, testa; labio multiplicato 



Mollusca trachelopoda, pallii marginibus in- "J . 



