6o Our British Snails 



water shell, swarming where found, e.g. from 

 Erith to Gravesend, and in East Anglia. Shell 

 small, thin, semi-transparent. 



Paludina [Paludestrina) jenkinsi. — A larger shell, 

 not confined to brackish water and spreading 

 very rapidly. Sw^arms where found. A variety 

 has a marked keel which sometimes bears bunches 

 of spines at equal distances. 



Paludina [Paludestrina) stagnalis. — Larger and 

 with more whorls. Not so common. 



Paludina (Pseudamnicola) anatina. — Small, sub- 

 conical, deep suture. Found in brackish water, 

 and apparently identical with Hydrobia or 

 Paludestrina similis, which I used to find by 

 the Thames, where it is now apparently extinct. 



Valvata piscinalis. — Globular, suture very deep, 

 circular mouth, operculum concentricall}^ spiral. 

 In ponds and slow water. Shell yellowish, but 

 commonly covered with conferva. 



Valvata cristata. — Much smaller ; shell disk- 

 shaped. Frequents the roots of flags. Shell 

 striated and more or less ridged, but the name 

 cristata refers to the plume-like appearance of 

 its breathing apparatus. 



We now come to the bivalve shells with leaf- 

 like gills. The Unionidse contain two genera, 

 Unio and Anodonta, commonly called fresh- 

 water mussels. 



Unio tumidus. — Shell ovate, very solid, dark 



