TROCHUS. 319 



J. G. J.) ; Dunbar, where " one specimen of this 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 

 Ins tow, 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 f Andrew) ; Santander, in 

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

 Trevelyan, Bart.) ; Mogador (M f 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- 



