XII 



PHYLUM MOLLUSC A 



683 



pre 



prs 



ridges, the hinge-teeth, separated by grooves or sockets, and so 

 arranged that the teeth of one valve fit into the sockets of the 

 other. 



The valves are marked externally by a series of concentric lines 

 (Fig. 562) parallel with the free edge or gape, and starting from a 

 swollen knob or elevation, the umbo {urn.), situated towards the 

 anterior edge of the hinge-line. These lines are lines of growth. 

 The shell is thickest at the umbo, which represents the part 

 first formed in the young animal, and new layers are deposited 

 under this original portion, as secretions from the mantle. As the 

 animal grows each layer projects beyond its predecessor, and in 

 this way successive outcrops are produced giving rise to the 

 markings in question. In 

 the region of the umbo 

 the shell is usually more 

 or less eroded by the action 

 of the carbonic acid in the 

 water. 



The inner surface of the 

 shell also presents charac- 

 teristic markings (Fig. 

 563, A). Parallel with 

 the gape and at a short 

 distance from it is a deli- 

 cate streak (pi. I.) caused 

 by the insertion into the 

 shell of muscular fibres 

 from the edge of the 

 mantle : the streak is 

 hence called the pallial 

 line. Beneath the anterior 

 end of the hinge the 

 pallia] line ends in an 

 oval mark, the anterior 



adductor impression (a. ad.), into which is inserted one of the 

 muscles which close the shell. A similar but larger posterior 

 adductor impression (p. ad.) lies beneath the posterior end of the 

 hinge. Two smaller markings in close relation with the anterior 

 adductor impression mark the origin of the anterior retractor (a. r.) 

 and of the protractor (pre.) of the foot : one connected with the 

 posterior adductor impression is that of the posterior retractor (p. r.) 

 of the foot. From all these impressions faint converging lines can 

 be traced to the umbo: they mark the gradual shifting of the 

 muscles during the growth of the animal. 



The shell consists of three layers. Outside is a brown horn-like 

 layer, the periostracwm (Fig. 564, pre), composed of conchiolin, a 

 substance allied in composition to chitin. Beneath this is a 



VOL. i x x 2* 



e.p.1 



ep.s. 



Fig. 564. — Vertical section of shell and mantle of 

 ^*1?fl9B* a — c - '• connective-tissue layer of 

 fiTantle ; ep. 1, its outer epithelium ; ep. 2, its 

 inner epithelium ; n. nacreous layer of shell ; 

 pre. pcriostracum ; prs. prismatic layer. (After 

 Clans.) 



