STRTTCTtmE AND PHYSIOLOGY OF THE MOLLUSCA. 



35 



which corresponds to it, shells cease to grow ; and these periodic 

 resting-places are often indicated by interruptions of the other- 

 wise regular lines of growth and colour, or by still more obvious 

 signs. It is probable that this pause, or cessation from growth, 

 extends into the breeding season ; otherwise there would be 

 two periods of growth and two of rest in each year. In many 

 shells the growth is uniform; but in others each stage is 

 finished by the development of a fringe, or ridge {varicc), or of a 

 row of spines, as in tridacna and murex. — {Owen, Grant.) 



Adult characters. The attainment of the full growth proper 

 to each species is usually marked 

 by changes in the shell. 



Some bivalves, like the oyster 

 and gryphcea (Fig. 26), continue 

 to increase in thickness long after 

 they have ceased to grow out- 

 wards; the greatest addition is 

 made to the lower valve, espe- 

 cially near the umbo ; and in the 



7 7 i. i? XT. j-i Fig. 26. Section of gryphcEa* 



spondylus some parts oi the mantle ^ 



secrete more than others, so that cavities, filled with fluid, 



are left in the substance of the shell. 



The adult teredo and fistulana close the end of their burrows ; 

 the pholadidea fills up the great pedal opening of its valves ; 

 and the aspergillum forms the porous disc from 

 which it takes its name. Sculptured shells, 

 particularly ammonites, and species of rostel- 

 laria and fusus, often become plain in the last 

 part of their growth. But the most charac- 

 teristic change is the thickening and contrac- 

 tion of the aperture in the univalves. The 

 young cowry (Fig. 27) has a thin, sharp lip, 

 which becomes curled inwards, and enormously 



thickened and toothed in the adult ; the ptero- 



ceras (PL 4, fig. 3) develops its scorpion-like 



claws only when full-grown ; and the land- 

 snails form a thickened lip, or narrow their 



aperture with projecting processes, so that it 



is a marvel how they pass in and out, and how 



they can exclude their eggs (e. g. PL 12, fig. 4, anastoma; and 



Fig. 5, helix hirsuta). 



* Fig. 26. Section of gryphcea incurva, Sby. Lias, Dorset (original; diminishecl 

 one-half) ; the upper valve is not much thickened ; the interior is filled with lias. 

 t Cypr/Ba testudinaria, L., young. 



Fig. 27. Young 

 Cowry.t 



