700 



ZOOLOGY 



SECT. 



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 in- 

 equivalved. Some Pseudolamellibranchs are, 

 however, nearly equilateral and markedly in- 

 equivalved, such as the scallop (Pcctcn), and 

 the inequi valve 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. 582, 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 aperture is closed by the flat 

 lid-like right valve : in Diceras, also extinct, 

 both valves are coiled. 



The hinge-teeth (Fig. 578) 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 

 modifications found in certain burrowing forms. In Pholas, a 



Fig. 580.— Solecurtus 

 strigillatus. s. af, 

 inhalant siphon, s. ef, 

 exhalant siphon, the 

 two united at SS. 

 (From the Cambridge 

 Natural History.) 



Fia. 581. — 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 ; f, second, between inhalant siphon and foot ; S, third, between byssal aperture and 

 foot. (From the Cambridge Natural History.) 



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

 brittle, and additional calcareous pieces are developed between 



