FAMILY 2. PERISTOMATA. 125 
Pl. CXCV. Fig. 5. 
MELANOPSIS FUSIFORMIS, Sowerby, Genera of Shells, No. 22. 
Melanopsis buccinoides (var.), Deshayes. 
Family 2. PERISTOMATA. 
Testa globosa, vel subturrita, epidermide olivacea induta, apertura feré 
rotunda, margine continuo. Animal fluviatile, operculo aut corneo, 
aut calcareo. 
The Peristomata are distinguished more by the similarity of their habit 
as living in fresh water than by any great similitude in their system of 
organization. It is, indeed, somewhat varied in these animals, though 
merely modified to the different necessities of their habits; like the Me- 
laniana, they have been variously distributed by authors, and we have 
not adopted the arrangement of Lamarck without some hesitation. The 
variation of character we allude to is in the Ampullarie, which exhibit a 
peculiarity of contrivance in their breathing apparatus found in no other 
moliusk. Being for the most part inhabitants of shallow stagnant pools, 
where the water is occasionally dried up, they are provided for this emer- 
gency by a double system of respiration, both a water-breathing, and an 
air-breathing system. By this arrangement they are preserved, and thus 
enabled to live a long time out of their natural element. 
The shell of the Peristomata may be described as being globose, or 
somewhat turrited, and covered with a dark olivaceous epidermis ; the 
aperture being generally round, with the margin continuous. The animal 
is fluviatile, and furnished with an operculum, which is either horny or 
calcareous. 
Three genera are referred to this family, as follows : 
VALVATA. AMPULLARIA. 
PaLUDINA. 
