PHYLUM MOLLUSCA 



651 



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 Pseudolamellibranchs are, 

 however, nearly equilateral and markedly 

 inequivalved, such as the scallop (JPecten), and 

 the inequivalve character is still more marked 

 in the oyster, in which the right valve is 

 deeply concavo-convex and permanently at- 

 tached 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. 544, B.), in which the right 

 valve has the form of a long tube closed at 

 one end by the flat lid-like left valve. In 

 the extinct Requienia- (A) the left valve is 

 spirally coiled so that it resembles a snail- 

 shell, and its aperture is closed by the flat 

 lid-like right valve : in Dicer as, also extinct, 

 both valves are coiled. 



The hinge-teeth (Fig. 540) vary indefinitely 

 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 

 lodifications found in certain burrowing forms. In Pholas, a 



FIG. 542. Solecurtus 

 strigillatus. . af, 

 inhalant siphon ; s. ej\ 

 exhalant siphon, the 

 two united at SS. 

 (From the C 

 Natural History.) 



l:o 



FIG. 543. 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 union between 

 siphons ; 2, second, between inhalant siphon and foot ; 3, third, between byssa aperture and 

 foot. (From the Cambridge Natural History.) 



, 



phonate genus which burrows in stone, the shell is weak and 

 brittle, and additional calcareous pieces are developed between 





