38 NELSON AND TAYLOR : ON YORKSHIRE MOLLUSCA, 



course of the season. The septa are formed in immature 

 individuals and are not a characteristic of adult specimens only, 

 as stated by some authors. 



Mr. Sheriff Tye, in his article upon mucous threads, denies 

 this species the power of making them, on account of its inhabit- 

 ing running streams. In Yorkshire this bar to the exercise of 

 the power does not exist, as the habitat in all cases is still pools. 



3. Vale of York — Dringhouses Bog, in profusion, W. C. Hey and others ! 

 This locality is sometimes, but not quite accurately, referred to as 

 'Askham Bog.' Tillmire ponds, a few in 1855, R.M. C. ; near 

 Osbaldwick, 1859, R.M.C. ; Hobmoor ponds, a few formerly, 

 R.M.C. ; Strensall Common, sparingly, W. C. Hey ; pond at Cop- 

 manthorpe, J. Emmet. 

 24. Holderness — Hornsea Mere, common, J.D.B, and W.D.R. ! 



Subgenus GYRAULUS Agassiz. 



Shell orbicular above, flat beneath ; whorls few, rapidly increasing (Adams' 



'Genera of Recent Mollusca,' ii. p. 263). 



Planorbis nitidus (Miill.). 



p. fragilis Mousson. 



Local and not common. 



The F. fragilis of Mousson is considered by Moquin- 

 Tandon to be a pale, thin variety of this species. 



The keel in the normal form is situate at or near the middle 

 of the shell, but Mr. Nelson has met with specimens at 

 Birmingham with the keel at the lower or basal edge as in 

 Flanorbis complanatus . 



It is closely allied to P. exacutus of Say, a species inhabiting 

 North America. 



The distribution of this species in Yorkshire is mainly 

 confined to the central and southern parts of the county, the only 

 exceptions being the four stations on the eastern coast. 



Trans.y.N.U., 1883 (pub. 1885). Series C 



