|0 NELSON AND TAYLOR : ON YORKSHIRE MOLLUSCA; 



Var. albina Jeffr. 

 22. Upper Denuent — Mr. C. Ashford has specimens sent to him by Mr. 

 Bean and stated to have been found at Scarborough. 



Sub-genus BULINUS Adanson. 

 Mantle simple-edged and not reflected over the shell. 



Bulirius^ like Segmentina, is sufficiently distinct to be con- 

 sidered a separate genus, but in this paper we retain them both 

 as sub-genera only. 



Adanson's name dating from 1797 has priority over the 

 names Aplexa and Nmita proposed by Fleming and Leach res- 

 pectively for this group. 



Physa hypnorum (L). 



Local and gregarious. 



Physa elongatina Lewis. 

 Physa glabra De Kay. 

 Bulinus tryoni Currier. 



The two names given above are referred to this species on 

 the authority of W. G. Binney and we refer Currier's species here 

 after a study of the description and figures given by Tryon. 



Of all our freshwater shells, P. hypnorum is the species 

 most strongly displaying the habit of making, and using for the 

 purpose of locomotion, the filaments of mucus which many of 

 our freshwater univalves are capable of exuding for this object. 

 The present species has been actually observed to form these 

 glutinous threads fourteen inches or more in length. 



This species is more local and gregarious than P.fontinalis, 

 and is perhaps the most hardy of our freshwater species, having 

 been found by Middendorf in Taimyrland, Siberia, in 73|-° 

 north latitude, but all Siberian specimens belong to Westerlund's 

 variety polaris, which is identical with the variety depressior 

 Middendorf, and differs from the type by its shorter and more 

 obese form and other characters. 



Trans. Y.N.U., 1S90 (pub. iSoi). Series C 



