ut a number of the particular ways in which the dynamics of
the environment may act on the characters of the hinge and shell of
bivalve molluscs. He has also shown that the initiation and development
of the columellar plaits in Voluta, Mitra, and other gasteropod molluscs
"are the necessary mechanical result of certain comparatively simple
physical conditions; and that the variations and peculiarities connected
with these plaits perfectly harmonize with the results which follow
within organic material subjected to analogous stresses."

In the same line of study is Dr. R. T. Jackson's[261] work on the
mechanical origin of characters in the lamellibranch molluscs. "The
bivalve nature of the shell doubtless arose," he says, "from the
splitting on the median line of a primitive univalvular ancestor;" and
he adds: "A parallel case is seen in the development of a bivalve shell
in ancient crustaceans;" in both types of shells "the form is induced by
the mechanical conditions of the case." The adductor muscles of bivalve
molluscs and crustaceans are, he shows plainly, the necessary
consequence of the bivalvular condition.

In his theory as to the origin of the siphon of the clam (_Mya
arenaria_), he explains it in a manner identical with Lamarck's
explanations of the origin of the wading and swimming birds, etc., even
to the use of the words "effort" and "habit."

"In _Mya arenaria_ we find a highly elongated siphon. In the young
the siphon hardly extends beyond the borders of the valves, and then
the animal lives at or close to the surface. In progressive growth,
as the animal burrows deeper, the siphon elongates, until it attains
a length many times the total length of the valves.

"The ontogeny of the individual and the paleontology of the family
both show that Mya came from a form with a very abbreviated siphon,
and it seems evident that the long siphon of this genus was brought
about by the effort to reach the surface induced by the habit of
deep burial."

"The tendency to equalize the form of growth in a horizontal plane,
or the geomalic tendency of Professor Hyatt,[262] is seen markedly
in pelecypods. In forms which crawl on the free borders of the
valves, the right and left growth in relation to the perpendicular
is obvious, and agrees with the right and left sides of the animal.
In Pecten the animal at rest lies on the right valve, and swims or
flies with the right valve lowermost