IN GENERAL. 687 



In many, especially bivalve shells, a horny, brown-yellow 

 outermost covering is found, which has been named epidermis or 

 recently periostracum also (in French Drop marin}. Sometimes it 

 is hairy or divided into scales, which however are more apparent at 

 the margin of the shell, having been worn off from the parts pre- 

 viously formed. This membrane has been regarded as a continua- 

 tion of the cuticle of the mantle, by which this is connected with 

 the margin of the shell. More correctly, perhaps, this covering may 

 be explained from a confluence of the intercellular matter, a resi- 

 duum of the formless homogeneous substance (the cytoblastema) *, in 

 which the cells filled with lime were formed. Let it be supposed 

 that at the outer margin of every layer this substance remains 

 without cells, and consequently hardens like horn. If these edges 

 should close upon each other, then a smooth epidermis will arise; if 

 they should remain more distant from each other, then a scaly, floc- 

 culent or hairy covering will be formed. 



The colours, presented by bivalve and univalve shells, are de- 

 posited only in their outermost parts, the inner layers are white. 

 This may be explained by the circumstance, that the colouring 

 matter is secreted especially by the edge of the mantle. But the 

 mantle grows with the animal, and thus each succeeding layer of 

 the shell is coloured at its outer margin alone, whilst its remaining 

 portion, secreted by the rest of the surface of the mantle, remains 

 white. In this way a series of coloured edges arises, which, closing 

 upon each other, form the outermost coloured surface of a shell. 

 There are however some univalves (the genus Cyprcea and some 

 Olivce] in which, when full grown, the colours are deposited not on 

 the surface alone, but also in a deeper layer, whilst at the same 

 time the superficial and the more deeply lying colours are different. 

 These molluscs are at first covered with a thin shell, of which the 

 colours must be ascribed to the edge of the mantle. As the animal 

 grows, lateral appendages of the mantle are developed, which throw 

 themselves like wings over the shell, and secrete on their outer 



Reports of the British Association for 1844 and 1847. The chief particulars of these 

 investigations may be also found in the Article Shell by the same writer in TODD'S 

 Cyclopedia, iv. 1849, P- 55^> & c - 

 1 Page 17, 1 8. 



