NELSON AND TAYLOR : ON YORKSHIRE MOLLUSCA. 59 



made with difficulty. The fluid is very probably the saliva 

 around the throat and stomach, or a peculiar humour contained 

 in its own special receptacle. Thishumourprobablyexiststhrough- 

 out the year, but is certainly present in April and September. 

 The fluid can be quickly obtained in great quantity, by throwing 

 into a linen bag plenty of the snails, and sprinkling a little salt 

 upon them, whereupon the red fluid is instantly and copiously 

 distilled. If powdered alum be sprinkled on the humour so dis- 

 tilled, the coloured part forthwith settles down, and the remain- 

 der becomes as limpid as water. If the experiment with salt 

 and alum be made in the same manner on the blueish 

 humour emitted from an incision by several terrestrial snails, 

 nothing coloured is thrown down, and the whole is hardened into 

 a blueish gluten. The coloured part of the fluid of P. corneus 

 can be separated with bibulous paper, but the beauty of the 

 colour perishes, changing to a dark sordid hue. If on the con- 

 trary it be mixed with vinegar or spirit of wine, or salt dissolved 

 out of dried cleansed plants, or common salt diluted not other- 

 wise than with alum, the bright red of this humour perishes. 

 When the fluid has been emitted voluntarily we have still in 

 vain tried to preserve it in narrow jars with well-closed mouths, 

 but with oil thrown over, or drenched with honey.' 



A variation to bright flesh or pink in the colour of the 

 animal itself was noticed by Mr. Nelson, near Leeds, during 

 the summer of 1878. The locality was a small cattle pond 

 near the Black Hills, one end of which sustains a dense growth 

 of reed, Elodea canadensis, Callitriche verna, Lemna minor, 

 and several freshwater algse, the opposite end being shallow and 

 quite destitute of plant life. The moUuscan inhabitants found 

 amongst the vegetation were a dwarf form of Sphcerium lacustre, 

 and an abundance of Pisidiutn pusillum and Flanorbis Jiautileus. 

 The shallow end of the pond was tenanted by the P. cornetis, 

 many of them having the animal pink or bright flesh colour, 

 rendering them very conspicuous. The peculiarity was to some 

 extent shared by the shells, which were much thinner, more 



