170
MUSEUM NOTES.
a short note (with a figure) in Science Gossip for 1896 (Vol. III.,
N.S. p. 147).
Mr. Kennard says that at Herne Bay it is by far the most
abundant mollusc, "living in burrows, about six inches deep, in
the London Clay. Associated with it is the well-known form,
Pholas candida which it (superficially) closely resembles, but the
'Pholas is by far the rarer. It is probable that the Petricola will
be the cause of the greater denudation of the foreshore, since its
burrows break up the London Clay, and thus enable the waves
to act in a more destructive manner than hitherto. There can
be no doubt that the mollusc has been established at Herne
Bay for many years, and that it has been overlooked through its
outward resemblance to Pholas candida. It may readily be
distinguished by the fact that the latter form possesses only one
siphon, whilst Petricola pholadiformis has two, and it was this
which first drew my attention to it. I believe that the species
will eventually become a common denizen of those parts of our
coast where there is a suitable habitat."'
The accompanying figures of Petricola are copied from the
Report of the Invertebrata of Massachusetts, by Dr. A. A. Gould,
edited by W. G. Binney (Boston, 1870). Although at first glance
the resemblance to Pholas candida is striking, it really belongs to
the Veneridae, a family most members of which are very unlike
Petricola superficially. The animals differ; in Pholas the respira-
tory siphons are united nearly to their ends; in Petricola they are
united only at the base. Pholas candida has a small plate (or
additional valve) over the hinge line and the ligament is not
visible externally. In Petricola the valves fit close at the hinge,
and there is a strong external ligament.
The habits of Petricola in America are similar to those
described above by Mr. Kennard. Dr. Gould says "On various
parts of our coast; at Chelsea and Nahaut beaches it is found
abundantly, imbedded in jutting fragments of a marsh which
once existed there, but which has been washed away by inroads
of the sea, and now only an occasional remnant lifts its head
above the surrounding sand. Also found in great quantities
boring into hard blue clay, at low-water mark on Phillip's
Beach."
It is an interesting biological question, how came it about
1 A. S. Kennard in litt.