1 72 POLYTROPA. 



these names as indicating growth modifications and localities : 

 those who take a less conservative view than myself will thus 

 have the names and descriptions at hand to designate these seve- 

 ral forms as varieties or species, or even genera, if it so please 

 them. I have also endeavored to illustrate a few of the transi- 

 tion forms. 



P. lapillus, Linn. PI. 52, tigs. 131, 134, 135, 137-144, 14, 147 ; 

 PL 53, figs. 148-151. Typical The metropolis of this form is 

 Northern Europe, the North American specimens, as well as 

 those from Southern Europe and North Africa, being stunted in 

 comparison of size and ornamentation. Its fossil distribution 

 ascends as far back as the Red Crag of England. It lives gre- 

 garious on rocks and stones within the tides, where it preys on 

 mussels, limpets, and barnacles. It is especially fond of oj'sters, 

 and is considered a destructive enemy by the cultivators of the 

 bivalve. A single reversed, as well as a scalaroid specimen are 

 recorded by Mr. Gwyn Jeffreys. He says that "this mollusk 

 has a shambling gait and sedentary habits, and seems to be 

 always eating or digesting its food. Lister, however, observed 

 it early in the morning, at the commencement of June, otherwise 

 engaged, viz., in perpetuating its species on a dry rock after the 

 tide had receded. It is very destructive to mussel-beds, and is 

 said by Linne to eat the dead fish left in fishermen's nets. 1 

 have seen it busily feeding on Balanu* balano'ides, its strong 

 proboscis being inserted between the opercular walls of the bar- 

 nacle. According to Mr. Osier, it also devours Littorinaj, Trochi, 

 Naticse, and even its own kind. From what I have observed of 

 the mode by which it perforates the shell of a mussel, I am in- 

 clined to agree with Mr. A. Hancock, that it uses its tongue. 1 

 cut off the end of the proboscis of a Pur pur a, while it was attack- 

 ing a mussel ; the part thus lopped still remains in the hole, with 

 the front of the tongue exposed. The hole is shaped like an in- 

 verted cone, and exhibits under the microscope extremely fine 

 scratch-like striae, as if caused by the rasping action of the lin- 

 gual apparatus. I believe the movement to be rotatory, because 

 the sides of the hole are quite even. The process is an extremely 

 slow one. Mr. Osier states that, after watching for some hours 

 a Purpura attached to a Limpet, he found the perforation incom- 

 plete ; and Mr. Spence Bate and Mr. Bretheiton noticed that it 



