22 NERITID&. 



valve or piece, which is more or less conical and spiral, and 

 usually covers the whole, but sometimes only the most vulnerable 

 part of the body ; it is either operculate or inoperculate. 



The land and freshwater univalves of this country 

 belong to one or other of the two following orders : 



I. PECTINIBRANCHIATA. 



II. PULMONOBRANCHIATA. 



ORDER I. PECTINIBRANCHIATA.* 



Body spiral ; respiratory organ a single comb-like gill, situated 

 inside the mantle on the upper side of the head. 

 Shell spiral, external. 



The freshwater univalves of Great Britain are com- 

 prised in three of the families which belong to this 



order, viz. : 



I. NERITIM;. 



II. PALUDINID^E. 

 III. VALVATID^:. 



The animals are provided with a pair of tentacles, 

 and two eyes placed at their base. 



The shell is operculate and covered by an epidermis. 



FAMILY I. NERITHX2B. 



Body short ; eyes placed on short foot-stalks at the base of 

 the tentacles ; gill seated within the mantle ; sexes separate. 



Shell semi-globose ; spire excentric, few-whorled ; mouth 

 transversely semi-lunar ; operculum provided on the under 

 side with a curved and deeply-grooved projection or plate-like 

 appendage. 



* Having comb-like gills. 



