12 The American Naturalist. [January, 
in favor of definite lines of variation, and in so far support the 
view that acquired characters are inherited. r 
Mr. Wm. H. Dall, in a recent highly interesting paper on — 
Pelecypods, explains the form and progressive development of the — 
ligament, cartilage pit, and teeth of that group as the result of the 
mechanical strains and stresses to which the parts ar exposed? — 
In the development of Pelecypods we find in a late embryonic — | 
stage (the phylembryonic) that the shell has a straight hinge line. 
This is characteristic of Ostrea (Fig. 1), Cardium, Anodonta, and so — 
many widely separated genera that it apparently represents a — 
primitive ancestral condition common to the whole class. Em- a 
bryolagy shows that the bivalve shell doubtless arose from the — 
splitting on the median line of a primitive univalvular ancestor, 
If that ancestor had a saddle-shaped* or a cup-shaped ‘ shell, as _ | 
is probable, the first result of the introduction of a hinge in the — , 
median line would have been to straighten the shell on the hinge — 
line. This isa simple problem in mechanics, for if one tries to — 
break by flexion a piece of metal which is saddle-shaped or cup- — 
shaped, it will tend to form a straight line on the axis of flexion. 
A parallel case is seen in the development of a bivalve shell in 4 
ancient crustaceans. The ancient Ostracoda: Leperditia, Aristozoe, — 
etc., have a straight hinge line and subcircular valves, which are 
united ay by a ligament. The resulting form of the early — 
condition of the bivalvular shell in 
these two distinct classes is so strik- 
ingly similar, it lends weight to our 
supposition that the form is induced 
by the mechanical conditions of the 
case. 
I think that the adductor muscles 
Wh. which close the valves may also be 
Fic. 1.—Ostrea edulis, embryo; demonstrated to be the necessary con- 
Sad anterior Zenon — ; sequence of the bivalvular condition. | 
hinge of shell; (after Huxley). ' In the phylembryo stage (Fig. 1) the 
valves are closed by a single adductor muscle, which is the sim- 
a Hinge of iniga Its Development, with an Attempt Toward a Better 
bdivision of the Group; by Wm, H. Dall. Am. Jour. Sci., Vol. XXXVIIL, Decu 
