NELSON AND TAYLOR : ON YORKSHIRE MOLLUSCA. 65 



OCT U iy45 J Genus PHYSA Lamarck. 



This group is a connecting link between Planorbis and 

 Limnsa, having amongst other peculiarities the setaceous tenta- 

 cles of Planorbis and the elongate spiral shell of Limn?ea, 

 though the latter is coiled dextrally and not sinistrally as in 

 Physa. The shells of the Physre are unusually brilliant 

 and glossy, which circumstance Dr. Jeffreys ascribed in some 

 degree to lubrication by the mantle lobes, apparently over- 

 looking the fact that of our two species, /'. hypnoruin usually 

 presents the most polished appearance, though quite deficient 

 of the extended mantle-lobes characterizing T'. /v//'/;;i?//>. The 

 Physse are very active moUusks and are the greatest adepts at 

 forming mucus-threads, which they freely use for ascending to the 

 surface or descending to the bottom of the water. The species 

 of this genus are said to be [)artiai to clear and shallow water. 



The genus is a very ancient one, species having been re- 

 corded from the Middle Purbeck Eed in tlie Secondary 

 formation. 



As in Planorbis, the species of this group do not ascend 

 generally the western dales, but principally congregate in the low 

 lying lands. 



Our two si)ecies belong to separate sections, which differ 

 by the presence or absence of the extended and digitate 

 mantle lobes, and in our o[)inion arc sufficiently distinct to be 

 elevated to full generic rank. 



I 



Physa fontinalis (L.). 



Locally coiiimon. 



Turbo adversiis Da Costa. 

 Lynin;va lal)rosa Risso. 



