PULMONIFERA. 173 



Tribe II. OPERCULATA.— with operculum. 



The Operculated Freshwater Cepkals are muck less numerous in 

 kind tkan tke Inoperculated. Tkere are only eleven species in 

 Britain. Tke operculum is an apparatus used ckiefly by water- 

 breatking mollusks ; and in tkose inhabiting tke sea it is developed 

 in great variety. Freskwater mollusks are comparatively few in 

 number, muck fewer indeed in Europe tkan in tke same iso- 

 tkermal latitudes of Nortk America ; and tke proportion that are 

 gilled and operculated, is smaller still. Our eleven species illus- 

 trate seven very ckaracteristic generic forms, two of wkick, Cyclo- 

 stoma and Acme, wkile possessing rudimentary gills, breatke air. 



Tke operculum is sometimes spiral, witk tke nucleus inclined 

 towards tke side, sometimes it is enlarged by concentric additions 

 witk tke nucleus in tke centre ; and it is eitker korny or superfi- 

 cially testaceous. 



Order I. PULMONIFERA— air-breathing. 



Respiratory organ a vascular sac for tlie respiration of air. 



Wken describing tke Pulmoniferous division of tke Inoperculated 

 Cepkals, we noticed tkat it embraced one-kalf of our entire series of 

 Land and Freskwater mollusks. Of tke Pulmoniferous Opercu- 

 lated Cepkals, we kave only two species in Britain. Tke pulmonary 

 organ, or lung, is a sac, over tke surface of wkick tke blood flows in 

 minute vessels for tke purpose of being aerated ; and it kas been 

 observed tkat in tke present Order tkey are closer togetker, in 

 parts forming flexuous ridges, wkick are supposed to be rudimen- 

 tary gills. 



Tke Pulmoniferous Operculated Cepkals are contained in one 

 Family : — 



1. Cyclostomacea. Animal kaving tke kead produced into a 

 ringed or ridged proboscis, tentacles bristle-like, witk tke 

 eyes at tkeir outer base, foot bearing on tke upper posterior 

 surface a skelly or korny operculum. 



