36 NELSON AND TAYLOR : ON YORKSHIRE MOLLUSCA. 



spring, the greater part of the old ones dying off before July, 

 and in many cases before June, or even May. 



Several of the species are found in a subfossil state in the 

 mud cliffs at Hornsea, and in a deposit which has evidently 

 been formed on the bed of an ancient lake at Askern. 



— :o:- 



Genus PLANORBIS Guettard. 



Dr. Jeffreys and Prof. Tate state that this genus has the 

 remarkable peculiarity of voluntarily emitting a purple fluid on 

 being irritated, this being considered a mode of defence against 

 their enemies, analogous to that of the Aplysia, stating that the 

 liquid is secreted by glands at the side of the neck. 



Professor Ray Lankester has however pointed out (' Zool. 

 Anzeiger,' p. 343, 1884), that the fluid is under no circum- 

 stances shed from the body of these animals unless the surface 

 be wounded. 



The aquiferous pores said to exist in the foot for the 

 admission of water to the blood-system, are denied by Prof. 

 Lankester to have any existence in fact. The expansion of the 

 foot would appear to be caused by a rapid flow of blood from 

 other parts of the body. 



The foot is short. The animals of this genus, when kept 

 in an aquarium, as well as naturally in ponds, are fond of crawl- 

 ing out of the water, also of floating on the under surface of the 

 water, and possess the power of making mucous threads, along 

 which they occasionally travel. 



The species of this genus do not appear in Yorkshire to 

 inhabit to any extent the Western dales of our county, and Mr. 

 J. S. Harper, of Sedbergh, informs us that no species of the 

 genus is found in that neighbourhood. 



:o: 



Subgenus SEGMENTINA Fleming. 



Shell orbicular, depressed, furnished with transverse testaceous partitions 

 or teeth. Aperture transversely oval or circular ( ' Adams' Genera of Recent 

 MoUusca,' vol. ii. p. 264). 



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



