110 CONCHOLOGICAL NOTES. 



The Periostraeum, fig. 8, is the horny cuticle, or 

 so-called epidermis. It is secreted by the free edge of the 

 mantle. It varys considerably in thickness ; on some 

 shells it is thin and soft, on others thick, coarse, or drawn 

 out into long, beard-like filaments. It protects the shell 

 from the action of erosive chemical substances, and the 

 adverse conditions of weather. 



The important internal features of a bivalve shell 

 are the anterior and posterior adductor muscle 

 impressions; the hinge-teeth; the pallial line and 

 sinus. In several genera of the BRACHIOPODA (which 

 have recently been isolated as a distinct PHYLUM) there is 

 a branchial skeleton, which, when in the form of a 

 loop, is supported by a calcareous process of the dorsal 

 valve ; but as a spiral form it is devoid of a support. 



The Muscle impressions indicate the points of 

 attachment of the adductor muscles. These muscles pass 

 through the body transversely, and by contraction, keep 

 the valves closed at the will of the animal, during which 

 time the cartilage is compressed. When the tension is 

 relaxed, the valves gape by reason of the expansion and 

 counter-action of the cartilage. 



The Hinge-teeth. These are of two kinds. The 

 cardinal teeth, which are situated immediately beneath 

 the uinbones, and those on either side of the cardinal, 

 named the lateral. Figs. 4, 4a, plate XIY. 



In the genus Area, the teeth are very numerous, 

 3542 ; in many other genera (Pecten Ostrea, and 



others), they are absent. 



The Pallial line, and the Sinus, figs. 2, 2a, 7, 7a. 

 The former is the line of attachment of the pallial muscle 

 of the mantle- edge to the shell, the curve (the sinus) in 

 its outline, is known as the pallial bay, or indentation, and 

 it permits the syphons, when retracted, to be contained 

 within the closed valves. 



