DREDGING IN THE RIVER CROUCH.
85
The other valve has an opening into which the "stopper" fits, and
by which it is attached to stones or other shells. On the south
coast it occurs abundantly of considerable size, on the shell of the
large Scallop (Pecten maximus) with both the valves moulded to the
conformation of the ribs of that toothsome mollusc. Some frag-
ments of wood were brought up riddled through with the borings of
a species of ship-worm, Teredo novalis (not really a worm, but a true
mollusc allied to the rockborers, Pholas, &c.); but in most cases
the brittle calcareous lining only was to be seen. On breaking the
wood up carefully, I was able to obtain the boring valves and a few
of the pallets, which enabled the species to be determined. The
valves are exquisitely sculptured in series of lines, and the pallets are
embedded nearly to the cusped tips, close by the long siphons.
The use of these pallets, which vary very greatly in the different
species, has never been clearly ascertained; but they are probably
used to close the tube and protect the mollusc when the siphons
are retracted.
A few good-sized specimens of Modiolaria marmorata, and the
larger form, M. nigra, were taken; and a large species which is called
the "Horse mussel" (Mytilus modiolus), which has a more hand-
some shell, well distinguished from the common mussel, not only
by the rich brown colour and thickish epidermis, but by the umboes
of the shell not being on the apex.
One dead valve of Cardium echinatum (the Horse Cockle),
showed that this exists in the Crouch, and we obtained many live
specimens of the small C. exiguum.
Another species of Tapes was also found, T. aureus, so called
from the yellow colour of the interior of the shell; but the shape
also is very different from the other species, the contour being more
wedge-like in form.
Some specimens of Scrobicularia alba came up; but, strangely
enough, S. piperita, which is larger, and the shells so common around
the coast at Walton-on-Naze, Harwich, Felixstowe, &c, did not seem
to be present—perhaps it is more purely marine than its smaller
ally ; or its absence may be due to the clearing of the bottom of the
river for the oyster culture. We also got two species of Sandgapers,
Mya arenaria and M. truncata. One of the latter, which I kept
alive some time, measured, when the siphons were fully extended,
53/4 inches.
Of the Gastropods (Univalves), we took two dead shells of