1890.] Studies of Pelecypoda. 1137 
velum at that stage, and be capable of swimming by means of 
that organ. In the nepionic stage the foot is of relatively great 
size, mobile and prehensile. It extends through a special notch 
in the right valve (Fig. 12), this being the side on which the foot 
exerts pressure in the act of crawling. This notch is a necessary 
mechanical consequence of the conditions of the case. At a 
later stage in Pecten we find that the early free crawling con- 
dition is abandoned, and the animal adopts the habit of byssal 
fixation, which is more or less adhered to until the animal is two to 
three centimetres in height ; but was not observed in adult Pecten 
irradians Although the swimming habit of Pecten irradians is 
Fic. 9.— Young Avicula sterna, umbonal area of valve, showing (5) prodissoconch 
and succeeding nepionic growth. 
Fic. 10.—Young Avicula sterna, showing later stage of growth. 
more or less exercised from the nepionic stages up, it is essentially 
the life-habit of the adult, for then the crawling and byssal habits 
are abandoned and the foot is atrophied. The swimming of 
Pecten is most active, and in the anatomy we find a most perfect 
adaptation of parts to the requirements of this peculiar habit. 
In the shell of Pecten (Figs. r1, 12) the prodissoconch is 
sharply defined, as in the genera already mentioned. It is 
directly referable in shell structure and in the inferred soft parts to 
to the nuculoid radical of the group. The nepionic stage (Figs. 
11, 12), is subrhombic in form, has no ears, and is a very striking 
period of growth. The right valve has a byssal sinus which 
originates with the nepionic stage as in Perna and Avicula, anda 
tooth exists on the margin of the notch. This stage is closely 
ish 
WELLES 
EEE jult sp f Chlamys islandi 
