682 



ZOOLOGY 



SECT. 



therefore equivalve. Each valve is inequilateral, being divided into 

 unequal portions by a line drawn from the umbo to the gape. It 

 will be remembered that in the Brachiopoda, the only other class 

 of bivalved animals, the precise opposite is the case, the shell being 



equilateral and inequivalved. Some Pseudo- 

 lamellibranchs are, however, nearly equilateral 

 and markedly inequivalved, such as the scallop 

 (Pecteri), and the inequi valve character is still 

 more marked in the oyster, in which the right 

 valve is deeply concavo-convex and permanently 

 attached to a rock, while the left is flat and 

 forms a sort of lid. This condition of things 

 reaches its maximum in the extinct Hippurites 

 (Fig. 592, B), in which the left valve has the 

 form of a long tube closed at one end by 

 the flat lid-like right valve. In the extinct 

 Requienia (A) the left valve is spirally coiled, 

 so that it resembles a snail-shell, and its aper- 

 ture is closed by the flat lid-like right valve : 

 in Diceras, also extinct, both valves are coiled. 

 The hinge-teeth (Fig. 588) vary greatly in 

 form and size or may be absent altogether : the 

 hinge-ligament is usually band-like, but in 

 Pecten takes the form of a cylindrical cord. 

 The variations in form, ornamentation, colour, 

 &c., among the many thousand known species 

 of shell are too numerous to mention ; but 

 reference must be made to peculiar modifica- 

 tions found in certain burrowing forms. In 

 Pholas, a siphonate genus which burrows in 

 stone, the shell is weak and brittle, and 

 additional calcareous pieces are developed between the two valves. 

 In Teredo (Fig. 593), the so-called Ship-worm, which causes great 



FIG. 590. Solecurtus 

 strigillatus. s. af, 

 inhalant siphon, s. ef, 

 exhalant siphon, the 

 two united at SS. 

 (From the Cambridge 

 Natural History.) 



FIG. 591. Diagram illustrating the various degrees of union of the mantle-lobes, b.o, byssal 

 aperture ; /. foot ; s. a, exhalant siphon ; s. b, inhalant siphon ; 1, first point of unioii 

 between siphons ; 2, second, between inhalant -siphon and foot ; 3, third, between byssal 

 aperture and foot. (From the Cambridge Natural History.) 



