876 THE AMERICAN NATURALIST. |. [Vor. XXXIV. 
The abnormalities of irradians also become significant in 
comparison with the normal condition in other species. The 
grooving of an external ridge takes place normally in P. island- 
icus, right valve, as we have seen. The development of a rib 
by interpolation in a groove is also typical of P. zslandicus, left 
valve. The formation of a small rib on the side of a typically 
large one is found on the left valve of P. zs/andicus, is the reg- 
ular thing in P. australis, from South Australia,! and is also 
common in other species where the number of rays increases 
with age. How are we to interpret this correspondence 
between an abnormality and a normal condition in another 
species? There was a time when we should not have hesitated 
to put the phenomenon in the category “reversion.” But we 
have so many instances of parallelism between the abnormal of 
one species and the normal of a second that we should be 
cautious in attributing them all to reversion. It seems better 
to recognize that a physiological potentiality which crops out 
(as an abnormality) in various species becomes fixed as a normal 
specific character in one of them. 
SUMMARY. 
The right or lower valve of P. irradians has on the average 
half a groove more than the upper, because the series of 
alternating ridges and grooves of the right valve has a prevail- 
ing tendency to end in grooves. Of three Long Island locali- 
ties, the most nearly land-surrounded shows Pectens with the 
greatest number of rays. The right valve is less variable than 
the left, a result which agrees with the fact that the right valve 
of Pectens is generally more archaic than the left. The varia- 
tion is nearly normal in both valves; more so in the left than 
in the right valve. The skewness is positive, showing a slight 
tendency towards an excessive production of the many-rayed 
individuals, or the selective annihilation of those with few rays. 
This positive skewness is paralleled by the fact that P. irra- 
dians seems to be developing towards a larger number of rays. 
The various abnormalities of Pecten are either explained as 
1 Field Columbian Museum, No. 8430. 
