Morse.] 364 [March 19, 
irregular lines of increment. The nucleus is oblong oval. The um- 
bones are nearly central, though nearer the anterior margin, and the 
shell is more globose behind. Both valves of the nucleus appear 
equaily convex, and no sign of a 
sinus or perforation is visible in 
either valve. On the free edge of 
the right valve, directly under the 
umbo, a distinct notch is seen, the 
lines of growth indicating it, and 
showing that the edge of the shell — 
is not absorbed to form this notch- 
It will be noticed that this mar- 
ginal notch appears in that valve 
which is below, and which after- 
wards presents the opening for the 
passage of the byssal plug. 
“The condition of the shell at 
this time clearly indicates that the 
animal is not only already attached, 
but has fallen to one side, and while 
in this position has added a few 
more lines of increment to its larval 
shell, as no sign of this notch is seen 
on the left or free valve. Soon, 
however, the peculiar and rapid se- 
cretion of a different shell growth 
takes place; the lines of increment 
are no longer regular, nor so con- 
Fig. 1. Fig. 2. 
Fig.1. Right or lower valve of Ano- 
mia, showing notch in ventral pallial 
margin, caused by byssus. Diameter one 
sixty-fourth inch. 
Fig.2. Left, or upper valve of Fig. 1. 
Fig. 3. Foramen commencing to form. 
The black portion shows proportions of 
left valve. 
Fig. 4. A more advanced stage of right 
valve with foramen almost completed. 
Fig. 5. Left, or upper valve of Fig. 4. 
Diam. one thirty-second inch. 
Fig. 6. Showing still later stage, with 
foramen completed, and nucleus still vis- 
ible. 
Fig. 5. 
Fig. 6. 
