XI] OF THE MOLLUSCAN SHELL 781 



the shell. And in not a few cases, as in Harpa, Dolium perdix, etc., 

 both ahke are conspicuous, ridges and colour-bands^ intersecting 

 one another in a beautiful isogonal system. 



In ordinary gastropods the shell is formed at or near the mantle- 

 edge. Here, near the mantle-border, is a groove Hned with a 

 secretory epithehum which produces the horny cuticle or perio 

 stracum of the shell*. A narrow zone of the mantle just behind 

 this secretes Hme abundantly, depositing it in a layer below the 

 periostracum ; and for some httle way back more hme may be 

 secreted, and pigment superadded from appropriate glands. Growth 

 and secretion are periodic rather than continuous. Even in a snail- 

 shell it is easy to see how the shell is built up of narrow annular 

 increments; and many other shells record, in conspicuous colour- 

 patterns, the alternate periods of rest and of activity which their 

 pigment-glands have undergone. 



The periodic accelerations and retardations in the growth of a 

 shell are marked in various ways. Often we have nothing more 

 than an increased activity from time to time at or near the mantle- 

 edge — enough to give rise to shght successive ridges, each corre- 

 sponding to a "generating curve" in the conformation of the shell. 

 But in many other cases, as in Murex, Ranella and the like, the 

 mantle-edge has its alternate phases of rest and of turgescence, its 

 outline being plain and even in the one and folded and contorted 

 in the other; and these recurring folds or pleatings of the edge 

 leave their impress in the form of various ridges, ruffles or comb-like 

 rows of spines upon the shell f. 



In not a few cases the colour-pattern shews, or seems to shew, 

 how some play of forces has fashioned and transformed the first 

 elementary pattern of pigmentary drops or jets. As the book- 

 binder drops or dusts a little colour on a viscous fluid, and then 

 produces the beautiful streamlines of his marbled papers by stirring 



* That the shell grows by accretion at the mantle-edge was one of Reaumur's 

 countless discoveries {Mem. Acad. Roy. des Sc. 1709, p. 364 seq.). It follows 

 that the mathematical "generating curves," as Moseley chose them, correspond to 

 the material increments of the shell. 



t The periodic appearance of a ridge, or row of tubercles, or other ornament 

 on the growing shell is illustrated or even exaggerated in the delicate "combs" 

 of Murex aculeatus. Here normal growth is. interrupted for the time being, the 

 mantle-edge is temporarily folded and reflexed, and shell-substance is poured out 

 into the folds. 



