280 FUNCTIONS AND INSTINCTS. 



have the same spiral configuration traced both on 

 the upper and lower surface. In most that I have 

 seen the intervals of the whirls increase in width, 

 as the spires of the shells do from the base to 

 the mouth. In the top-shell 1 the whirls are 

 perfectly regular and nearly equidistant. They 

 vary much in thickness ; I have one three-fourths 

 of an inch thick, while those of the top-shell 

 and periwinkle are very thin. In some of the 

 thick ones, on the under side the convolutions 

 are very convex, and sometimes elevated into 

 concentrical ridges. Some underneath have a 

 forest of obtuse elevations, and many are rough 

 with minute tubercles. As to substance some are 

 horny, while others resemble the shell ; others 

 are horny externally and shelly internally. If 

 these formations on the under side, as in the 

 common periwinkle, represent the shape of the 

 part of the neck to which they are attached, 

 as they most probably do, it must act the part 

 of a mould, upon which the operculum is formed 

 from its mucus, and increased as the aperture 

 enlarges. 



Lamarck is of opinion that the shell of uni- 

 valves is formed in a similar way upon the neck 

 of the animal, which in the Murices or rock- 

 shells, and other tribes distinguished by spines 

 or tubercles, has certain fleshy processes which 



1 Trochus 



