No. 407.] PECTEN IRRADIANS LAMARCK. 871 
lies close by the side of one of normal size (Fig. 2, 7). All 
these abnormalities may be regarded as congenital. In addition 
I have found certain variations, probably due to injury of the 
mantle. Jackson (90) remarks that the mantle is subject to 
injury by fish and thereafter regenerates only imperfectly. He 
accounts for the incurved edge of the shell which is occasionally 
found (Fig. 1, 7) as follows: “When one mantle border is 
injured, the other repairs the damage which would be caused 
by local want of shell growth, by curving the shell deposition of 
the injured side rapidly inwards, thus obviating the deficiency 
of the injured area. This ingrowth is probably induced by the 
lack of resisting support on the part of the damaged border." 
We have now to consider the question whether the individual 
variations and the abnormalities which we have studied throw 
any light on the question of the origin of species in the Pectin- 
ida. This involves an examination of the genus Pecten.! 
This genus has been variously subdivided. The subgenus 
Chlamys (as defined by Dall, '90, p. 695), which includes P. irra- 
dians, is a fairly well defined subdivision, and our attention 
may be confined to it. The species all have * ribs," as I shall 
designate the alternating ridges and furrows, which are true 
flutings or corrugations affecting both inner and outer surfaces 
of the shell. If the ribs are absent in any case, they have 
secondarily become lost. The valves are nearly equally in- 
flated; the ears vary from a nearly equal condition to one 
of great inequality. The anterior ear of the right valve is 
typically notched, to let pass the byssus. In judging of the 
position of P. irradians we need to know something of the 
phylogeny of the subgenus Chlamys. We can infer some- 
thing, as Jackson (90) has, on the recapitulation hypothesis, 
by noting the condition of the shell at different stages of 
development. The record of these changes remains in the 
adult shell, and can be studied by examining the beak and 
adjacent parts. Fossil remains also tell something. The beak 
! In making this examination I have had the great privilege of examining, with 
the kind assistance of Dr. S. E. Meek of the Field Columbian Museum, the very 
fine collection of Pectens possessed by that institution. 
