TROCHUS. 319 



J. G. J.) ; Dunbar, where '' one specimen of tins shell 

 was taken by the dredge from deep water ''■' (Laskey) ; 

 Peterhead (Macgillivray) ; Cumbrae, Clyde district 

 (J. Smith). Da Costa also gives Hampshire and 

 Norfolk; but these and the Scotch localities want con- 

 firmation. With similar hesitation I must cite my 

 friend Mr. Smith as the authority for considering this 

 species fossil in the Paisley beds. The variety is from 

 Instow, North Devon (J. G. J.), and Arran Isles, co. Gal- 

 way (Barlee). The typical form inhabits the north of 

 France (De Gerville and others) ; Rochelle (J. G. J.) ; 

 Vigo, and Faro in Algarve (M' Andrew) ; Santander, in 

 the north of Spain (E. J. Lowe) ; Hyeres (Sir W. C. 

 Trevelyan, Bart.) ; Mogador (M^Andrew and R. T. 

 Lowe) . 



The motion of the foot is wave-like, each side alter- 

 nately. On leaving the water this Trochus takes in a 

 supply of air, which (if the animal be again immersed) 

 is expelled or escapes in bubbles by the right-hand 

 lappet of the mouth. The erosion of the shell, which 

 is not unfrequent, seems to be caused, and is certainly 

 increased, by the perforations of a minute kind of sea- 

 weed or its spores ; water enters the orifices thus formed, 

 and gradually effects a disintegration of the outer layers, 

 one after another. The whole fabric not being of a 

 homogeneous nature, or equally compact, some parts 

 are more easily acted on than others. Mr. Clark found 

 that every specimen in a particular spot near Exmouth 

 had a distorted operculum ; this was irregularly pauci- 

 spiral, and sometimes ear-shaped, but always had a 

 central nucleus. He accounts for it in the following 

 way. '' The animal either sheds the operculum, or is 

 deprived of it by the attacks of enemies, perhaps from 

 its own pulli, white masses of which, in the genial sea- 



