58 NELSON AND TAYLOR: ON YORKSHIRE MOLLUSCA. 



Subgenus CO RET US Adanson. 



Shell depressed, not keeled, slightly umbilicated above, smooth, whorls 

 increasing somewhat rapidly ; the last moderate in size, without interior 

 lamellaB ; aperture broadly semilunar (Moquin-Tandon, ' Hist. Nat. d. 

 Mollusq. terr. et fluv. de France,' ii. p. 445). 



Planorbis corneus (L.). 



Not uncommon in the lowlands. 



Many of the recorded localities of this species, especially 

 near the towns, are ponds or ditches, where they have been 

 introduced by conchologists or others, being a favourite 

 aquarium shell. Wherever we have been able to obtain definite 

 information about its introduction to any pond or other place, 

 we have stated the fact when quoting the locality. 



Mr. Wilcock found numbers of subfossil specimens near 

 Conisborough in 1873, and in May, 1885, Mr. W. D. Roebuck 

 found a few with P. cornplanatus, &c., in a bed on the banks of 

 the River Tutt, two miles south of Boroughbridge. One speci- 

 men was obtained by Rev. E. P. Knubley, in 1885, on the site 

 of an old pond at Staveley. P. corneus is also numerous at 

 Askern, thrown up by moles or exposed by the ploughing of 

 the land. 



Dr. Martin Lister, as long ago as 1678, gave a full account 

 of the red fluid secreted so abundantly by this species, which 

 we think of sufficient interest to reproduce here : — 



' It emits a red humour if a little salt of any kind, or a 

 little pepper or ginger, be placed in the aperture of the shell. 

 It is doubtful whether this really comes from the body like 

 blood from a cut, though I have seen it emitted from a tentacle 

 broken off close to the head, and also to flow from a slight 

 puncture of the body. Experiments that would establish with 

 certainty whether the fluid comes from a wound or not are 



Trans.Y.N.U., ISSi (pub. 1886). Series C 



