272 SHELL DREDGING. 
within its limits, and the marine species alone (containing 
all the animals that belong to the branch of mollusca, though 
many have no hard calcareous shells) number three hundred 
and sixteen. Several of these are cuttle-fishes, and there 
are many mollusks which have no shells, the branehism or 
gills being’ naked; hence they are called Nudibranchia. 
They comprise the most beautiful animals in the branch of 
Mollusca, for certain species are very brilliantly colored. 
The species figured on the plate are among the few that 
the collector is likely to bring up while dredging in our bays 
and inlets, in depths of from ten to fifteen fathoms. Should 
he be ambitious to throw his dredge into depths of fifty or 
one hundred fathoms, many other species will be secured 
that he could not get in water of less depth. 
The outlines given will be found sufficiently accurate to 
enable the collector to identify the species represented. Fig. 
1 represents Chiton albus ; the shell is not quite half an inch 
in length; it is generally a dead white color. Figs. 4 and 5 
represent Velutina haliotoides and V. zonata, the latter dif- 
fering from the former in having a more solid shell, and 
in having the shell marked with bands of brown. Fig- 6 is 
the Natica immaculata, a pure white shell of the size repre 
sented; very common. Fig. 13 represents another species, 
Natica clausa; color from a white to a dark reddish brown. 
The little lid that closes the aperture of most marine shells 
is in this species white and shelly, and not of the horny 
consistency characterizing the opereula of most shells in our 
region. ` Pandora trilineata (Fig. 24) is easily distinguished 
by its white pearly color, and the manner in which the valves 
are pressed together. Lyonsia hyalina (Fig. 20) has a vety 
fragile translucent shell eovered with radiating wrinkles. — 
Thyasira Gouldii (Fig. 18) has a delicate white shell, along” 
one margin of which is a deep fold. The shell of Astarte — 
. castanea (Fig. 22) is quite solid, and chestnut-colored. It 
_ is found abundantly in Provincetown harbor at low wa 
- Astarte sulcata (Fig. 25) is known by its strongly marked - 


