LAMELLIBRANCHIATA. 



the apex is usually shorter, and contains, at least in the equivalve 

 species, an excavation, the lunula, by means of which the anterior 

 edge can be at once recognised. 



While the outer surface of the shell presents various sculpture 

 markings, the inner surface is smooth and shines with the lustre of 

 mother-of-pearl. On a closer examination, impressions and pits become 

 visible on the inner surface. A narrow line, the so-called mantle or 

 pallial line (the line of attachment of the mantle edge to the shell), is 

 placed near and fairly parallel to the ventral edge of the shell (fig. 

 499, Ml). In the siphoned forms this presents posteriorly a bend 

 directed forwards and upwards (Mb), the pallial bay, which is due to 

 the siphons. Impressions are 

 usually caused by the inser- 

 tion of an anterior and pos- 

 terior adductor muscle which 

 pass throitgh the body of the 

 animal transversely from 

 one side to the other, and 

 are attached to the inner 

 surface of the shell (fig. 

 499, HM, VM). While in 

 the equivalve mussels (Or- 

 thoconcha) the two impres- 

 sions are usually of equal 

 size, in the unequivalve 

 forms (Pleuroconcha) tne 

 anterior adductor is re- 



FIG. 501. Vertical section through the shell and 



dliced, and may Completely mantle of Anodonta (after Leydig). Cu, cuticle ; 

 vanish; the posterior ad- f P^matic layer; ^, laminated (mother-of-pearl) 



layer; Sp 1 , external epithelium of mantle; Ld, 

 ductor, On the Other hand, connective tissue substance: Ep", internal epi- 



now a muscle of much larger melium of mantle ' 



size, shifts forward to the middle of the shell (fig. 500). Hence 

 the names Dimyaria, and Monomyaria. According to its chemical 

 composition, the shell consists of carbonate of lime and an organic 

 matrix (conchyolin), which usually presents a laminated texture. 

 In addition to this laminated layer there is also a thick external 

 calcareous layer, composed of large, pallisade-like prisms, which 

 are placed side by side and may be compared to the enamel of 

 teeth (fig. 501, ). Finally, on the outer surface of the shell 

 there is a horny cuticle, the so-called epidermis (Cu). The internal 

 laminated layer is secreted by the whole surface of the mantle, 



