" SHELLS " 307 



in old days, there was considerable trade among the ancient Britons. Of 

 univalves a considerable number of species occur in British waters, the com- 

 monest being Limncea stagnalis, to be found in every stagnant pool or ditch. 

 It has a thin shell, spirally coiled, conical in shape, i| to 2 inches long. It is 

 frequently seen travelling, shell downwards, along the under surface of the 

 water. If kept in an aquarium, its eggs will often be seen in small gelatinous 

 masses attached to weeds, or even to the glass. 



The Fresh-water Limpet (Anchylus fluviatilis) is no near relative to the 

 marine limpets, but gets its name from its similarity of appearance. It is 

 very small, the shell seldom more than J inch in length. It is found in streams. 

 A similar species, but smaller still (A. locustris), is found in still water. 



The Pond Snail, RAM'S HORN, or TRUMPET SNAIL (Planorbis corneus), has a 

 large flat spirally coiled shell. It is quite common. When disturbed it dis- 

 charges a purple-coloured fluid, probably as a means of defence as the Cuttle- 

 fish eject their ink. It is a vegetable feeder. 



Paludina vivipara is common in the South of England. It belongs to a 

 species which breathes by means of gills. Its name (vivipara) reminds us that 

 it retains its eggs inside the body until they are hatched. The shell is over 

 an inch long and nearly as wide, and has an operculunt. 



Bithynia is a genus having shells | to inch long, provided with opercula. 

 B. tentaculus, the commonest, has a shell of about six coils. Frequent in 

 canals and ditches. 



Marine Shells. 



Probably every visitor to the seaside has begun as a child to collect the 

 shells to be found on every shore. My space is becoming limited, and I must 

 confine myself to a brief description of some of the commoner species. 



UNIVALVES. 



1. The Common Whelk (Buccinum undatum). This species is responsible 

 for the clusters of yellowish white eggs found on the seashore, and often called 

 " sponges," varying in size from a small to a large cricket ball, and of a parch- 

 ment-like texture. When first laid they are very small, no bigger than a pin's 

 head, but they rapidly swell as soon as water reaches the inside. This species, 

 though common enough, is only apparent on the beach in the shape of its large 

 empty shells, for its habitat is from low-water mark to a hundred fathoms. 



2. The Netted Dog-whelk (Nassa reticulata] is much smaller. The shell 

 is coloured brownish yellow outside, pinkish white within, and is covered with 

 broad grooves, crossed by fine lines at right angles, producing a netted appear- 

 ance. It is both a vegetarian and a carnivorous feeder. It possesses a tooth- 

 ribbon of nearly 1,000 teeth. 



3. The Sting Winkle (Murex erinaceus] is white or yellowish white, about 

 i| inch long, with several high notched ridges. It has no " sting," but it 



