Zoology. 733 
bility that apposition of organic elements may occur on the inner 
y surface of the shell, at those places where the shell is perma- 
nently united with the body, z. e., from the muscles. The outer 
margin of the shell, that is the thickened periosteum and the 
inner surface next the mantle, are always soft. The calcification 
both of the prismatic and mother-of-pearl layers, is due to small, 
roundish, irregularly distributed bodies, which gradually increase 
a in all dimensions and become prismatic by mutual pressure. 
During the metamorphoses of the young mussel the shell has 
a fibrillar structure; the lamellation is secondary, probably be- 
ginning along with the calcification. The original fibrillar struc- 
ture is associated with the development and differentiation of the 
shell-muscles. The organic substance of the shell has a cellular 
origin, 
In their development the fibrils follow the directions of the 
mantle-muscles. They assume a radial course at the ligament, 
but elsewhere run parallel to the surface of the mantle, following 
the direction of the muscle-fibers which run transversely round 
the animal, just under the epithelium, and which, uniting with the 
tooth-pad, the pallial line and the periosteum, thus exert influ- 
€nce on the fibrils. 
These transverse muscle-fibers aid in the opening of the shell. 
Those radially disposed on the back of the animal flatten the 
ligament in contracting, and thus also aid in opening, as those 
also do which ascend on each side from the foot, and are at- 
tached to the tooth or tooth-pad. The muscle-fibers uniting 
the dorsal muscle insertions on either side, act as adductors. The 
7 bundles of cross muscles on the margin of the mantles, which are 
: by one end attached to the shell on the pallial line, and the other 
k to the free portion of the periostæum, effect by their contraction 
the apposition of the soft-shell margins, and a consequent perfect 
a Closure of the shell.—/Journ. Ry. Micr. Soc., June, 1886. 
valve of the shell to a firm board and connecting the other by a 
of 
silk thread with the short arm of a lever, the longer arm of which | 
ce kly. A — 
ton of its proper ganglion, each muscle can be studied sepa= 
rately 
The author sums up his conclusions as follows: “ Two classes — io 
‘Rerve-fibers supply the adductor muscles—(@) motor, causing 
