438 MARINE INVERTEBRATES 
P. hastatus. There are several very beautiful pectens on the Pacific 
coast of the United States, and two of these—one of northern and the other 
of southern range —resemble very closely the two Atlantic species of 
relatively north and south range, P. islandicus and P. irradians or dislo- 
catus. The first is P. hastatus of Puget Sound. Valves of this exquisite 
shell are even occasionally found in California. It has very much the 
aspect of P. wslandicus — 
unequal ears and an ovate 
form. The most curious 
thing about this shell is the 
fact that a different system 
of sculpturing exists upon 
the two valves. Upon one 
the ribs are even and the 
whole surface is covered 
with raised scales; upon 
the other the ribs are alter- 
nately of large and small 
size. Color rose-pink, with 
deeper bands; one valve 
lighter-colored, _ suffused 
with delicate pink. Height 
two and a half inches. 
(Plate LX XIX.) 
P. equisulcatus. The 
Southern species above re- 
ferred to as resembling P. 
Pecten irradians, typical form. trradians or dislocatus. It 
has about twenty stout, 
well-rounded ribs, with concentric wrinkles between them. The “ ears” 
are equal. The size and shape are asin P. irradians. The coloring is 
whitish, varying to shades of brown, and excentrically marked with 
spots or waving transverse lines of chestnut or horn. ; 
ORDER EULAMELLIBRANCHIATA 
With the pectens we leave behind all pelecypods with the 
true filamentous type of gill, In the order Eulamellibranchi- 
ata we find all the families to be possessed of the basket- 
work or crisscross branchial structure. Each gill is fun- 
damentally formed of filaments which project downward, and 
then suddenly turn upon themselves and ascend; the ascending 
and descending limbs are connected, however, by processes of 
presumably vascular function, and adjacent filaments are con- 
nected by truly vascular channels. The closer connection of the 
filaments to form of each gill a sort of plate is further effected 
