866 THE AMERICAN NATURALIST. [VoL XXXIV. 
so I counted these grooves to get the laws of variation in the 
number of radii in Pecten. Such a study is of some impor- 
tance, because the valves on which the series occur are not 
typically symmetrical, but show a certain tendency towards 
being dorsal and ventral We have here, then, a case among 
pelecypods resembling that of the flounder among fishes. 
The grooves in all the shells examined ran continuously 
from near the beak to the free margin of the shell. I did not 
observe any case of bifurcation of a groove, the obliteration of 
a groove distally, or the introduction of a new groove towards 
the free margin of the shell This is the more remarkable 
because in allied species, such as P. eboreus Conrad (see Dall, 
'98, p. 749), some of the ribs become obsolete toward the 
margin, and in others, as P. zs/audicus, new grooves are begun 
at various stages in development of the shell. 
The number of grooves corresponds to the number of ridges 
in the *middle" area. But the number of grooves on the 
inner face of the shell is not always definite, especially towards 
the transition area. We here sometimes find a very slight 
lateral depression passing hinge-ward only one-tenth to one- 
fifth the whole distance. The rule was adopted to count as 
grooves only those depressions where the margin of the 
shell was folded to form a complete U rather than merely 
an L. A strictly ambiguous or halfway groove at both ends 
of the series counted for one full groove, but when only 
one groove was ambiguous, that was omitted from the count. 
About ten per cent of the shells from Cold Spring Harbor and 
all from the other two localities were counted by my wife, 
Gertrude C. Davenport; the rest were counted by myself. 
We satisfied ourselves of the equivalence of our estimates of 
doubtful cases by making a series of independent determina- 
tions on a number of identical shells, finding that we agreed in 
our determinations in the case of each individual shell. 
The shells from Cold Spring Harbor were picked up on the 
sand spit, where they had been thrown by waves and shifting 
sands. They were therefore not pairs, and were not con- 
sciously selected. The shells from Fire and Oak Island Beaches 
were picked up on the beach and were also separate and 
