No. 407.] PECTEN IRRADIANS LAMARCK. 875 
tend to larger numbers than irradians. In many of these 
cases of species, with a larger number of rays at the periph- 
ery, there is an ontogenetic increase. Thus the epionic 
shell of miniaceus has only sixteen rays, and the left valve 
of islandicus has only nineteen ridges at 5 mm. from the beak. 
These species, then, judging from ontogenetic changes, have 
been derived from species with fewer rays, such as we find in 
irradians. Finally, Dall (98, p. 748) concludes in respect to 
P. gibbus ( — irradians in part): ** Taking all varieties together, 
the generalization may fairly be made that in the Pliocene the 
proportion of specimens with less than nineteen ribs is decid- 
edly larger than among recent shells." The apparent contra- 
diction between this statement and the figures which Dall gives 
for the number of rays in fossil groups suggests that the fig- 
ures are based on too few individuals to be significant. It may 
be concluded, consequently, that the condition of about eight- 
een rays exhibited by P. zrradiass is not far removed from 
the ancestral condition, and that most of the species with 
numerous rays have been derived from forms which, like irra- 
dians, have fewer than twenty rays.! 
These facts have a close relation to those of individual varia- 
tion in our form-unit of P. irradians. First, the number of ribs, 
which is so variable in the individuals of the form-unit studied, 
is likewise very different in the different species of the genus. 
Again, as we have seen, the asymmetry of groove frequen- 
cies in P. irradians is in the positive sense; that is, there is a 
tendency to an excess of rays; in other words, there is a tend- 
ency to vary in the direction of P. zs/audzcus; to go the path 
that it has trod. 
1I have paid some attention to the ways in which the increase in rays is 
brought about in the different species. In islandicus the nineteen rays at the 
beak increase in the left valve to forty, chiefly by the interpolation of new ridges 
in the old furrows. Such interpolated ridges start at various stages. In the older 
shells the increase is also effected by bifurcation of certain of the larger ribs, of 
Which there are about six to eight. In the right valve, on the other hand, the 
ribs increase chiefly by bifurcation, although interpolations also occur. I have 
observed the same difference between the left and right valves in other species ; 
namely, P. alvolineatus Sby., from Viti (Field Columbian Museum, No. 6111); 
P. tigrinus Müll., from Great Britain (Field Columbian Museum, No. 6118); and 
P. madreporarium Petit, from Singapore (Field Columbian Museum, No. 6109). 
