870 THE AMERICAN NATURALIST. [Vor. XXXIV. 
varying environmental conditions. This small variability of 
the lower valve is in accord with the fact that the young shell 
of Pecten is larger and better preserved on the right valve than 
on the left. Again, in P. squamosus, in which the scales 
are becoming obsolete in the adult, they are found at a later 
stage on the right valve than on the left. In other cases,” 
the grooves of the left valve divide and become ornamented, 
while the right valve remains simple. . Here, then, the index of 
variability is an index of phylogenetic changeableness. 
5. The type of distribution of frequencies was determined 
only for the shells from Cold Spring Harbor, because they alone 
were sufficiently numerous for this purpose. The right valve 
is Type IV, with a rather small skewness however, namely, 
+0.023. For the left valve the distribution is remarkably 
near the theoretical normal distribution, the skewness being 
+.0000000058. The skewness will rarely be theoretically 
zero. Applying Pearson’s limits, the distribution may be said 
to be of the normal type. Applying the prevailing method of 
interpreting these results, we may say that the Cold Spring 
Harbor race is, as regards the grooves in the upper valve, very 
stable; while as regards the grooves in the lower valve it 
outer. exhibits a slight tendency to the exces- 
caue VO cae TE EN sive production of a large number of 
i rays, or to the selective annihilation 
i "X. Lu. of the small numbers. The excessive 
c. "wA production of large numbers may be 
. due to innate or to external causes. 
= ud N Abnormalities of the grooves often 
inue. occur. All gradations were found be- 
Fic. 2— Cross-Sectionsof de i; tWEEN the condition of a broad ridge 
maa seo fitus : tbe on the inner face of the shell and the 
a double ridge; o a double COndition of a double ridge, the halves 
Grooves d,a lateral groove. — of which were separated by a groove 
(Figs. 1, a, 4; 2,6). In other cases the groove contains à ridge 
of varying size (Figs. 1, c; 2, c). Occasionally a small groove 
1 From Mauritius, No. 2412, Field Columbian Museum 
? For metes Pecten madreporium, from Singapore, No. 6109, Field Colum- 
bian Mus 
