OUR FRESH-WATER MOLLUSCA. 159 



a noble and common species^ and both the shell and animal will well repay 

 your observation. Those two compressed species, which look hardly thicker 

 than an ordinary card, are respectively named Vortex and Spirorhis, and it 

 requires a good eye to discriminate between them. The other species, 

 especially P. nifidus, the rarest of the genus, affect deeper water. F. mar- 

 (jinafua and P. carinatus, two closely-allied species, rejoice in the cool 

 under- surface of a water-lily leaf. They may be most easily distinguished 

 by the dark colour of the animal of P. marginatus. 



Yonder fine shell, tapering up into a lofty spire, which also floats shell 

 downward, is called Limneus stagnalis, and is common in the middle and 

 south of England. It is found, though rarely, as far north as York. In 

 Oxfordshire it is sometimes more than two inches in length, and specimens 

 from the Danube are said greatly to exceed ours in size. 



If you examine the wet mud by the side of your pool you will probably 

 find two other Limnei — palustris and truncatulus, both of which are common 

 though interesting moUusks. Limneus auricularius, a handsome and large 

 shell of the same genus, is crawling on the mud at the bottom. As that 

 large leech undulates past him, he shrinks, as though alarmed, into his 

 shell; and with reason, for your leech, who is a thorough cannibal, and 

 can rarely gratify his taste for human or animal blood, will not unfre- 

 quently make up the deficiency with the soft and shrinking body of the 

 unfortunate water-snail. I have caught him in the act. 



In the deepest part of the pool you may, if you are fortunate, find a 

 specimen of the beautiful and rare Amphipeplea glutinosa, whose shell is 

 enveloped in a curiously- spotted, slimy mantle. When it is extracted there- 

 from, it is of most delicate and beautiful texture. It is found, though 

 rarely, near Oxford, and, more abundantly, in Norfolk. 



Nor are bivalves, which form so beautiful a part of the Marine Shells, 

 unrepresented in our pool. Stir up the mud with a stick, and when the 

 water has cleared, you will find the gaping shells of various species of 

 Gyclas and Pisidium, which have left their beds to see what the ^'troubling 

 of the waters" has done for their support. Cyclas lacustris is as delicate 

 and beautiful a bivalve as you can see, and G. rivicola is a handsome shell 

 — very abundant in the sluggish Oxfordshire streams. The genus Pisidium 

 is difficult to understand. Perhaps the best solution is that of the naturalists 

 who affirm that there are only three species — amnicum, pulchellum, and 

 pusillum. I have reason to know that one of the best living authorities 

 holds this view. 



You will doubtless find many other species which I need not now enu- 

 merate. Y'^our apparatus for searching is simple — a piece of coarse canvass 

 on a hoop, at the end of a pole, wilt make as effective a fresh-water dredge 

 as you can desire. A small ladle of wire-gauze made to fix on the end 



