124 ANATOMY OF THE LAMELLIBRANCHIATA. 



of this colouring matter. From its easy destructability it has 

 been supposed not to be of a mineral nature. ■ The coloured 

 markings on the external surface of the valves are of various 

 forms ; they are dependant upon the disposition of the veins 

 of the mantle ; they are often interrupted from the cessation 

 of the secretion at certain periods. The internal stains some- 

 times seen, and which pervade the whole thickness of the 

 shells, are produced by the contact of an excreting venous 

 organ, 2 destined to throw off the redundant colouring and 

 earthy matter, &c, and from its secretion the foot and extre- 

 mities of the tubes also are often brilliantly stained. Light 

 has an effect on this coloration ; when one valve is fixed, or 

 is constantly buried in the sand, the other, being most exposed, 

 is most coloured ; and such species as live immured in the 

 interior of rocks, wood, &c, are commonly destitute of colour. 

 The articulation of the valves, one with the other, presents 

 an infinite variety. The elastic substance, or cartilage 1 is 

 so placed, as to be compressed when the valves are closed by 

 the muscles ; and, regaining its original state when the mus- 

 cular force ceases, to open the shell. In addition to the car- 

 tilage, a ligament frequently adds to the security of the hinge. 

 These two may be conjoined or not. The cartilage is often 

 divided. In Pema it is perfectly so, the portions being situ- 

 ated in parallel grooves. In Area, &c, the portions are con- 

 joined at the point of the beak and diverge from it. The 

 former appears to be the divided elongated cartilage seen in 

 the generality of the Dimyaria ; the latter the divided verti- 

 cal cartilage, common in the Monomyaria. The cartilage is 

 composed of layers like the shell, being secreted, when inter- 

 nal, in a corresponding sac of the mantle, or, when external, 

 by a glandular prominence of it from a set of minute glands. 

 The former is the case in the Pecten, Spondylus, &c, the 

 latter in the Anadonta, Bucardium, &c. In the former case 

 the layers are deposited from below ; in the latter from behind. 

 There is no case in which the cartilage is before the beaks, 

 unless, as in the Area, it is divided and divergent. In all 

 cases, the cartilage must have its commencement apparent 

 at the very beaks of the valves, unless eroded, as it is in Os- 

 trea, Gryphcea, &c. This disunion of the cartilage may take 

 place more from one valve than the other, as is seen in some 

 species of the last named genus, causing the great length of 



1 Iodine and bromine have been found in these shells. Is either of 

 them concerned in the coloration ? 



2 Not of the liver, as supposed by Blainville. 



3 Gray has shown the distinction between the cartilage and the ligament, 

 1 Zoolog. Journal,' vol. 1. 



