400 THE ANATOMY OF INVERTEBRATED ANIMALS. 



always two large pallial lobes, the margins of which are de- 

 void of setae ; and which are lateral, or right and left, in rela- 

 tion to the median plane. Each .lobe gives rise to apiece, or 

 valve, of the shell ; and to these, accessory pieces, developed 

 upon the median haemal face (Pholas) or the posterior end of 

 the mantle (Teredo), are in some cases added; or, in addition 

 to its valves, the mantle may secrete a shelly tube (Teredo, 

 Asperg ilium). The shell itself consists of superimposed 

 lamellae of organic matter, hardened by the deposit of calca- 

 reous salts. It is a cuticular excretion from the surface of the 

 mantle, and never presents any cellular structure. But, from 

 the disposition of its lamellae, and from the manner in which 

 the calcareous deposit takes place in them, it may present 

 varieties of structure which have been distinguished as nacre- 

 ous, prismatic, and epidermic. 1 



The two valves are generally united over the median line 

 of the haemal surface of the body by an uncalcified chitinous 

 cuticular matter, termed the ligament, which is usually very 

 elastic, and is so disposed that, when the valves are closed, it 

 is either stretched or compressed. In either case, it antago- 

 nizes the action of the adductor muscles, and divaricates the 

 valves when these muscles are relaxed. Conch ologists com- 

 monly draw a distinction between an internal and an external 

 ligament ; but, in relation to the body of the animal, all liga- 

 ments are external, and their internality or externality is in 

 respect of the hinge-line, or the line along which the edges 

 of the valves meet. In symmetrical, or equivalve, Lamelli- 

 branchs, each valve is concave internally and convex exter- 

 nally; it has, in fact, the form of a very depressed cone, the 

 apex of which, termed the umbo, is incurved and is situated 

 on, or projects beyond, the haemal, or, as it is termed, dorsal 

 edge of the valve. Moreover, it is usually inclined forward, 

 and situated nearer the anterior than the posterior end of the 

 valve. Sometimes the umbonic cone is prolonged and bent 

 inward, or may even form a short spiral turn (Tsocardia, 

 Diceras), so that the valve acquires a certain resemblance to 

 the shell of some gasteropods. As the shell of a Lamellibranch 

 increases in thickness by the deposition of new layers on the 

 interior fice of the old ones, and, in area, by the extension of 

 the new layers beyond the old ones, the summit of the umbo 

 represents the original shell of the embryo, and the outer sur- 



1 See Carpenter, article " Shell," Todd's " Cyclopaedia." Huxley, " Tegu- 

 mentary Organs," ibid. 



