20 The American Naturalist. (January, 
of totally distinct form in the free young. The several types. 
cited are from widely separate families of Pelecypods, yet all 
under the same given conditions, adopt a closely similar form, 
which “is strong proof that common forces acting on all alike 
have induced the resulting form. What the forces are that 
have induced this form it is not easy to see from the study of 
this form alone; but the ostrean form is the base of a series, from 
the summit of which we get a clearer view. 
As I stated in an earlier paper," the fullest modification in the 
ostrean line of variation is the production of a shell in which the 
attached valve is cup-shaped, conical, or subcylindrical, as seen 
markedly in species of the Chamide and Rudiste. In this 
group as a whole, and in progressive stages of growth of its 
extremest members, all steps may be followed between a simple 
ostreiform or exogyriform shell and the most highly modified 
conical type. The Ostrea form is the first step in this line of 
modification, the Exogyra form is the second step, and the 
conical form is the last step. What are the mechanical causes 
which bring about this resultant form? I suggest as an hypoth- 
esis the following: The Rudiste are conical or cup-shaped 
Pelecypods, with a superficially marked radial symmetry. So 
striking is the radial feature that they have been classed with the 
corals or Cirripeds, and the term radial is combined frequently 
in generic and specific names of the group. Barretia monolifera, 
as described by Woodward, is highly radial, and the infoldings 
of its thin walls closely resemble the radial septa of corals. In 
other animals which are permanently attached by calcareous fixa- 
tion, as corals, some worms and Brachiopods, Cirripeds, and 
others, we find closely comparable forms which are subcylindrical 
or subconical, with a very marked degree of radial symmetry. 
Finding so many similar forms built up on different lines of 
descent affords strong evidence that common forces acting on all 
alike have induced the resulting form. The equal impact of mov- 
ing water on all sides of an attached, growing organism, it seems, 
would cause an equal effort of resistance on all sides, and there- 
fore induce an equal growth on all sides, thus producing a form 
11 Studies of Pelecypoda, AMER. NAT., Dec. 1890, p. 1135. 
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