872 THE AMERICAN NATURALIST. [Vor. XXXIV. 
of the valves, especially the rzght valve, shows an area with- 
out striations ; at this stage also the hinge is as long as the 
greatest diameter of the shell, and there are no sharply marked 
off *ears." At this stage the shell resembles the Devonian 
Aviculoids, which are usually regarded from this as well as 
from paleontological evidence as the ancestors of the Pectinidae. 
The genus Pecten began to emerge in the Devonian and Car- 
boniferous. These emerging Pectens, Pterinopecten (Devonian), 
had a long hinge line, with large, not sharply defined, nearly 
equal ears, and with a byssal sinus on the right valve. Both 
the valves seem to have been convex. Jackson (90, p. 386) 
says that Ptrerinopecten dignatus Hall and other species bear a 
close resemblance to the young of P. irradians. In Aviculo- 
pecten (Devonian), which represents another step towards Pec- 
ten, we see the hinge becoming shorter and the two ears well 
defined; a deep byssal sinus occurs on the right valve in many 
species. In both Pterinopecten and Aviculopecten the shell 
is corrugated. These dawning Pectens then had the charac- 
ters of the subgenus Chlamys, which may consequently be 
regarded as the most primitive of the subgenera of Pecten. 
P. irradians, therefore, belongs to the most primitive division 
of the genus Pecten. 
Inside the subgenus Chlamys the species show modifications 
in various directions. There are species in which the posterior 
ear has become very much smaller relatively than in irradians ; 
e.g. islandicus of our coast, hericeus (= hastatus) of the Cali- 
fornia coast and Vancouver, and niveus of Great Britain. 
Associated with this diminution in the posterior ear is the 
formation of scales in a linear series on the ribs. The scaled 
condition is secondary, as is clearly shown in niveus, where 
the younger shell is without scales ; 1 the scales are obtained 
only in the later stages. The inequality of the ears is also 
derived, for in the primitive condition there was equality of 
the auricles. This group is therefore more modified than 
P. irradians. 
1 The scales appear earlier on the lower than on the upper shell. 
2It is interesting that in certain species, ear, P. sguamosus of Mauritius, 
scales seem to be secondarily disappearing. This species would seem to have 
originated from an ancestor resembling P. hericeus. 
the 
