BIVALVE SHELLS. 55 



power of movement also is extremely limited, their cells not 

 being cylindrical, whilst one of them, Saxicava, is fixed in its 

 crypt b}^ a b^^ssus. These shell-fish have been supposed to 

 dissolve the rock by chemical means (Deshayes), or else to wear 

 it awa}^ with the thickened anterior margins of the mantle 

 (Hancock), or b}^ the incessant action of the cilise of the mantle 

 margin ( Agassiz). All attempts to prove the use of an acid 

 secretion in excavating have hitherto failed, as might be expected ; 

 for the hypothesis of an acid solvent supposes onl}^ a very feeble 

 but continuous action, such as in nature always works out the 

 greatest results in the end. 



The holes of the Lithodomi often serve to shelter other 

 animals after the death of the rightful owners ; species of 

 Modiola, Area, Venerupis, and Coralliophaga, both recent and 

 fossil, have been found in such situations-, and mistaken for the 

 real miners. 



The boring bivalves have been called "stone-eaters" (litho- 

 phagi), and "wood-eaters" {xylophagi) , and some of them at 

 least are obliged to swallow the material produced by their 

 operations, although they may derive no sustenance from it. 

 The ship-worm is often filled with pulpy, impalpable sawdust, of 

 the color of the timber in which it worked (Hancock). No 

 shell-fish deepens or enlargies its burrow after attaining the full 

 growth usual to its species. 



Between the bivalve mollusks having attached valves, as in the 

 oyster, and those which are free, as in Cardium, Lucina, Tellina, 

 the fresh-water mussel, etc., are placed not only those in which 

 the liberty of the animal is circumscribed by its living in a 

 burrow, self-immured for life in a cell ; but also those which spin 

 a byssus (iii, 40), by which the shell is attached to some foreign 

 substance, from which the individuals of the species usually 

 depend in clusters or colonies. Usually byssus-spiuners, as the 

 Modiolas, Mytilus, or sea-mussel, are inhabitants of shore lines, 

 in situations where, at low tide they are exposed above the 

 waters ; they are thus subject to the wash and lifting power of 

 the incoming waves, and but for their anchor-like fastenings 

 would be torn away from their haunts, dispersed, and frequently 

 destroyed — as most free shells are in similar situations. 



The valves of the Conchifera are bound together by an elastic 

 ligament (ii, 31), and usually articulated by a hinge furnished 

 with interlocking teeth (ibid.). The shell is closed by powerful 

 adductor muscles (ibid.), but opens spontaneously by the action 

 of the ligament, when the animal relaxes, and after it is dead. 



Each valve is a hollow cone, with the apex turned more or less 

 to one side ; the apex is the point from which the growth of the 

 valve commences, and is termed the beak, or umbo {ibid.). The 

 beaks (umbones) are near the hinge, because that side grows least 



