686 ON MOLLUSCS 



is suspended. Hence stronger stripes arise, which indicate succes- 

 sion in growth, and are the traces left of it. 



As to chemical composition, besides some other less constant 

 constituents and a small quantity of phosphate of lime, univalve 

 and bivalve shells consist principally of carbonate of lime and an 

 animal substance, a membranous substratum, that remains after the 

 calcareous matter has been dissolved by acid, and is usually very 

 small in quantity compared with the calcareous matter. POLI, 

 when he exposed the membrane to the fire, saw it quickly take 

 flame, on which he perceived a smell like that of burning horn; a 

 spongy carbon remained. 



This conducts us to a correct view of the nature of shells, 

 which belong to the same tissue as horn, hair, and in part also 

 scales, i.e. to horny tissue. In most animals the dermal skeleton 

 (such is the name given to the hard parts placed externally, to 

 which the muscles are attached,) is horny, whilst, on the other 

 hand, cartilage is almost always the foundation of an internal 

 skeleton, especially of a true neural skeleton. The microscopic 

 structure of bivalve and univalve shells has been chiefly illustrated 

 by the investigations of CARPENTER. In some bivalves the entire 

 substance of the shell consists of layers of membrane, without 

 visible cells, in others such a membranous tissue forms the inside of 

 the shell, whilst on the outside columnar, often hexagonal cells are 

 visible under the microscope, which to the naked eye, or with the 

 use of a lens, look like fibres. They stand nearly at right angles 

 on the surface of the shell from within outwards, and are filled 

 with carbonate of lime. In each layer they are at that part only 

 which projects beyond the edge of the preceding layer; thus they 

 have been secreted by the edge of the mantle, whilst the whole 

 mantle, in every new formation of a layer, produces a membrane 

 that covers the inner surface of the entire shell. The univalve 

 shells of the gasteropodous molluscs have only a small quantity of 

 organic substance; in many three layers of plates may be distin- 

 guished; the direction of these plates is different, and those of the 

 middle layer intersect those of the external and internal at right 

 angles. Every plate consists of a row of long columns, or pris- 

 matic cells, which are arranged side by side 1 . 



1 See W. B. CARPENTER Annals of Nat. History, Vol. xii. 1843, PP- 377 39' 

 PI. XIII. XIV. and especially his later, more general investigations, announced in 



