MOLLUSCA. 15 
the mantle alone which provides for the increase 
of the shell in superficial extent. On examining 
“this part, it is found to be of a glandular character, 
and moreover not unfrequently provided with a de- 
licate and highly sensitive frmge of minute tenta- 
cula. Considered more attentively, it is seen to 
contain in its substance patches of different colours, 
corresponding both in tint and relative position 
with those that decorate the exterior of the shell. 
‘When the animal is engaged in increasing the 
dimensions of its abode, the margin of the mantie 
is protruded, and firmly adherent all round to the 
circumference of the valve with which it corre- 
sponds. ‘Thus circumstanced, it secretes calcareous 
matter, and deposits it in a soft state upon the 
extreme edge of the shell, where the secretion 
hardens and becomes converted into a layer of solid 
testaceous substance. At intervals this process is 
repeated, and every newly-formed layer enlarges 
the diameter of the valve. The concentric strata 
thus deposited remain distinguishable externally, 
and thus the lines of growth marking the progres- 
sive increase of size may easily he traced. 
“It appears that at certain times the deposition 
of calcareous substance from the fringed circum- 
ference of the mantle is much more abundant than 
at others: in this case ridges are formed at distinct 
intervals; or, if the border of the mantle at such 
periods shoots out beyond its usual position, broad 
plates of shell, or spines of different lengths, are 
secreted, which, remaining permanent, indicate, by 
the interspaces separating successively deposited 
growths of this description, the periodical stimulus 
to increased action that caused their formation. 
‘« Whatever thickness the shell may subsequently 
