14 



MOLLUSC A. 



of the older and half-decayed layers are still attached to the 

 bottom of the furrow, and half fill it, as in Fig. 12, h" . 



The horny portions are evidently produced around the 

 extreme edges of the mantle margins everywhere, and by 

 the narrow area along the hinge. 



The so-called skin or epidermis of the shell, which covers 

 the outer surface of a valve with a continuous layer, can be 

 peeled off, when present, with a knife-blade. It is a horny 

 layer produced by the extreme edge of the mantle, which, 

 in living and fully-expanded animals, are capable of stretch- 

 ing even farther outwards than would be necessary for this 

 work. 



The ligament or hinge is continuous, layer by layer, 

 with this epidermis, and is evidently a part of the epider- 

 mal secretions, though originating from a distmct region 

 of the mantle. 



It is composed of three parts : two external parts com- 

 posed of loose layers, which are continuous with the horny 

 layers of the edge of the mantle, and a denser, central part 

 continuous with the latter, and of the same striated or 

 columnar structure (Figs. 5 and 12, Ji). 



For obtaining a view of other parts, lift the outer 

 part of the mantle on the convex border, and turn it 

 back. 



This shows below the four gills (Fig. 2,g). The 

 scholars can readily trace these to both ends, and find 

 that they are united along their bases, and terminate 

 both ways ; at one end where the leaves of the mantle 

 are united (Fig. 2, ind), and at the other where four 

 broad, pointed flaps reach out and embrace their ex- 

 tremities when the animal is in perfect condition and 

 at ease (Fig. 2,// Fig. 12). The fluted or frilled char- 

 acter, of the gills, and their serrated edges, can be seen 

 with the naked eye (Figs. 8,/^/, and 15, and 6'). With 



