PELECYPODS 423 
two and a half inches in length and one and a half inches in height, 
and is dark olive-green. The foot is like that of Y. limatula. The 
labial palps are exceedingly large. 
Y. sapotilla. The shell is 
ovate, prolonged on one side, 
thin, fragile, translucent, and 
covered with a thin, glossy epi- 
dermis, greenish in color. The 
anterior half is semi-oval, the 
posterior portion narrowed and 
compressed ; within it is pearly- 
white, with a triangular cartilage 
cavity, and 
sixteen to eigh- 
teen pointed == 
teeth on each Yoldia thracicyormis. 
side. In length 
it is little less than an inch. Found from Long Island 
Sound northward. 
Yoldia sapotilla, 
FAMILY sOLENOMYIDE 
Genus Solenomya 
Two species of this genus, which are greatly prized by collectors 
on account of the extraordinary development of the epidermis of 
the shell, occur in New England waters. The strong corneous 
periostracum of a deep chestnut-color projects considerably be- 
yond the margins of the valves; the shells are exceedingly thin 
and fragile, and are marked with radiating 
lines. The two species are easily distinguished 
by their difference in size. S.velum is about 
one inch long and one half of an inch high. 
S. borealis, of arctic range, is quite twice that 
size. Their anatomy is similar in essentials to that of Yoldia. 
These species live near shore in sandy bottoms, and are occa- 
sionally found upon the beach between tides. Chelsea Beach, 
just north of Boston, is an often-quoted locality. 
ORDER FILIBRANCHIATA 
There are several families of this order which are well repre- 
sented in both the Atlantic and the Pacific waters of the United 
States. The characteristic feature of this group of bivalves is the 
. filamentous gill, that is, a gill with the filaments long, doubled 
back, and united to each other only by ciliary junctures. These 
Solenomya velum, 
