KEY TO FAMILIES. .7 



fresh waters of all parts of the world. It is 

 numerously represented in the United States. 



Order SCUTIBRANCHIATA. 



Sub-order Podopthalma. 



Includes marine, and a few fluviatile species 

 (Neretinye). Head furnished with tioo tenta- 

 cles and with eyes on separate pedicels. Shell 

 spiral, operculate. 



Family Neritid^. 



Shell generally semi-globose, spiral of few 

 whorls, not umbilicated. Cavity simple from 

 the absorption of the internal portions of the 

 whorls. 



Distribution tropical and sub-tropicah 



Order PECTINIBRANCHIATA. 



Sub-order Rostrifera. 

 Includes marine and fluviatile species. Head 

 with a produced, contractile rostrum. Tenta- 

 cles subulate. 



