322 
THE OCEAN WORLD. 
abode, the margin of the mantle is protruded and firmly adherent a 
round to the circumference of the valve with which it correspon s - 
Thus circumstanced, it secretes calcareous matter and deposits it upon 
the extreme edge of the shell, when the secretion hardens and become 3 
converted into a layer of solid testaceous substance. At intervals A 1 ' 
process is repeated, and every newly-formed layer enlarges the ia 
meter of the valve. The concentric strata thus deposited remain dis- 
tinguishable externally, and thus the lines of growth marking t 
progressive increase of size may easily be traced.” 
“ While the margin of the mantle is thus the sole agent in enlarging 
the circumference of the shell,” the professor continues farther nu> 
“its growth in thickness is accomplished by a secretion of a kind o 
calcareous varnish derived from the external surface of the rnant 
generally, which, being deposited layer by layer over the whole ulte- 
rior of the previously existing shell, progressively adds to its weigh ■ 
and solidity. There is, however, a remarkable difference in character 
between the material secreted by the marginal fringe and that ft 1 ' 
nished by the general surface of the mantle membrane. The foi®^ 
we have found more or less covered by glands appointed for the P 111 ' 
pose, situated in the circumference of the mantle ; but as these gla® ' 
do not exist elsewhere, no colouring matter is ever mixed with t ® 
layers that increase the thickness of the shell, so that the lat ^ 
always remain of a delicate whitish hue, and form the well kno^ 
iridescent material usually distinguished by the name of nacre 0 
mother-of-pearl.” ( General Outline, p. 385.) g 
The process by which shells attain their beautiful markings is th u 
described by Mr. Jones : — “ The external surface is exclusively dep° 
sited by the margin of the mantle, which contains in its substa» ce 
certain coloured spots, which are found to he of a glandular character 
and to owe their peculiar character to a pigment they secrete, 
is mixed with the calcareous matter ; coloured lines are therefore f°^ 
on the exterior of the shell wherever these glandular organs eXl A. 
Where the deposition of colour is kept up throughout the process 
enlargement, the lines are unbroken and perfect; but where 
coloured matter is furnished only at intervals, spots and patches 
irregular form and increasing in size with the enlargement of 
mantle are the consequence.” ^ 0 f 
Bivalves move about and change from place to place by mean. 
