STUtTCmiE AND PHYSIOLOGY OF THE MOLLUSCA. 



35 



Section of gryphteo 



'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 fi'oni 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 [varix), 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 

 spondyliis some parts of the mantle 

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

 are left in the substance of the shell. 



The adult teredo and fistidana 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. ^. PI. 12, fig. 4, anastoma; and 

 Pig. 0, helix hirsuta). 



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

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

 t Cyprcea testudinaria, L., young. 



Fig, 27. Young 

 Cowry, t 



