516 LECTURE XXI. 



properly) the " cartilage," is lodged in the furrows between the 

 ligamental plates, or in pits along the hinge-line ; it is composed of 

 elastic fibres placed perpendicularly to the surfaces of attachment, so 

 as to be compressed by the shutting of the shell, which they con- 

 sequently tend to open as soon as the action of the adductors ceases. 

 The two parts are very distinct in the bivalves thence called 

 Amphidesma (double ligament) ; but coexist in most genera, with 

 alternate proportions, the ligament being small in Mactra which has 

 a large spring, and large in Anodon which has a small spring. Tlie 

 Pholades have the spring, but have not the ligament: this is replaced 

 by the homologue of the anterior adductor, which is so situated as 

 to act as an opener of the shell, and is called the " umbonal muscle." 

 The functions of the shell in this boring bivalve are too active 

 and too frequently in exercise to be performed by the passive elastic 

 antagonist of the muscular closing powers which sufl3.ces for or- 

 dinary bivalves. 



The formation and repair of the shell are due to the development, 

 change of form, and calcification of cells from the mantle, its whole 

 outer surface being the matrix of the nacreous layer, its thick and 

 periodically glandular margin that of the opake outer portion. 



The microscopic structure of bivalve and univalve shells has been 

 well illustrated by Profs. Carpenter* and Quekettf. The primitive 

 nucleated condition of the cell is sometimes retained after calci- 

 fication. J The dissolved lime-salts, after endosmotic penetration of 

 the organic walls of the modelling cell, obey so far the general 

 crystallising force as to polarise light. The forms of the constituent 

 lime-particles of the shell, so moulded by combined vital and polar- 

 ising forces, are manifold in the various genera of bivalves. The 

 shell of the Pinna, save a thin internal layer, is composed of vertical, 

 slender, usually hexagonal prisms. A thin outer layer of the shell 

 of the oyster also presents the prismatic cellular tissue ; but in a 

 great proportion of this shell nearly all trace of development from 

 cells is lost : the gelatinous basis is lamelliform, and this variety is 

 called the subnacreous shell-substance. 



Fine tubes, analogous to those of dentine, permeate the thickness 

 of this substance in many shells ; radiating vertically between the 

 ribs in Area ; vertical and scattered in the inner layer, and reticulate 

 in the outer foliaceous spines, of the shell of Chama, which has an 

 intermediate layer of ill-defined vertical prisms. The prismatic 

 structure is rarely found, and then only in a small proportion, in 

 the bivalves which have the mantle lobes in any degree united. 



* CCCX. t CCXXXL X lb. p. 270., fig. 157. 



