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. Pott, 
when he exposed the membrane to the fire, saw it quickly take 
flame, on which he perceived a smell like that of buming 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, ze. 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 night 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 See W. B. Carpenter Annals of Nat. History, Vol. x11. 1843, pp. 377—399, 
Pl. xu. xiv. and especially his later, more general investigations, announced in 
