LITTORINA. 373 



never gritty^ which last would certainly be the case if it 

 contained testaceous fry. It is sometimes striped like 

 the zebra. In one individual which I examined the 

 right-hand tentacle was branched like a stag's antler 3 

 and Dr. Johnston mentions a specimen '' in which the 

 tentacula were divided into two branches. ^^ In another 

 individual the left-hand tentacle had been mutilated^ 

 and appeared not to be of more use to the periwinkle 

 than the stump of an arm would be to a crippled soldier, 

 who had lost that limb on the field of battle. Besides 

 the monstrosities or malformations above specified, and 

 which appear to have resulted from some injury sus- 

 tained by the mantle, the shell is liable to be affected 

 by chemical action and other causes. On one part of 

 the shore of the Thames at Southend I found almost 

 every specimen of L. litorea more or less eroded, some 

 of them to so great an extent as to be distorted. This 

 could not have been owing to the admixture of fresh 

 and salt water, because on another part of the same 

 shore, where a stream flowed into the sea, none of 

 the specimens which I found were eroded. In many 

 places on the open coast, where there is no fresh water, 

 all the shells, as well as the limestone rocks, are fretted. 

 An explanation of this curious phenomenon was ofi'ered 

 in the Introduction (pp. 1-liv) to the first volume of 

 this work. Shells thicker than usual are often attacked 

 and penetrated, sometimes by minute AlgcBy and at other 

 times by a species of Clionay or by a small cylindrical 

 annelid ; the latter frequently destroys the upper whorls. 

 One specimen in my collection is so encrusted with 

 bleached nullipore as to be easily mistaken for a small 

 lump of chalk. I have a pearl which was extracted from 

 the common periwinkle ; it is round and white, the tenth 

 of an inch in diameter. Petiver noticed the large size of 



