290 NOTES ON PALUDESTRINA JENKINSI.
varieties have been described. In 1890 Mr. Jenkins, with Mr.
Smith's sanction, considered the carinated form to be the type
and named the uncarinated form var. ecarinata. In 1891 he
described var. tumida (Science Gossip, vol. xxvii., p. 9), and in the
same year he proposed a var. gracilis, which, however, is fortu-
nately still undescribed. The shell is a very variable one, the
non-carinated examples passing into the carinate and the tumid
form into the tall slender one, so that these names are of no
use. Mr. L. E. Adams has considered the smooth form as the
type and has recorded a var. carinata, Smith, but this is an
obvious error (op. cit., pp. 144-5).
In spite of the fact that this species has not been found
outside the British Islands, it has been suggested that it is an
introduction, and Mr. L. E. Adams has enunciated the theory
that it has come from Finland with timber. As we have
already shown it is a widely distributed form in these Islands,
and discontinuous distribution is in itself almost sufficient to
prove that it is an ancient inhabitant. In 1807 our friend Dr.
Frank Corner sent us a small box of shells which he had obtained
from a section exposed in enlarging one of the "fleets" in the
Roding Valley, near Barking. The shells occurred in patches
under two to three feet of "marsh clay." There were about a
dozen examples of Paludestrina jenkinsi associated with Bythinia
tentaculata (Linn), Limnaea truncatula (Müll.), Planorbis marginatus
(Drap.), and P. spirorbis (Linn.). These shells still retained
their periostracum, a characteristic of many of the shells from
the Alluvium. It is, of course impossible to pronounce defi-
nitely on the age of these shells, but they are of considerable
antiquity though within the historic period. Thus there can be
no doubt that Paludestrina jenkinsi has lived in the Thames Estuary
for a very considerable time.
In 1859 the late Mr. G. B Sowerby figured, but did not
describe, a shell under the name of Rissoa castanea, Jeffreys,
examples of which had been taken by Mr. Pickering in a ditch
about two miles below Gravesend.4 Dr. J. Gwyn Jeffreys, in
referring to these examples, states that they were considered by
Forbes and Hanley, though with some doubt, to be a variety of
Hydrobia ventrosa, but, in his opinion, since they so greatly resembled
a species of Hydrobia from the Cape of Good Hope, he could not
4 Illustrated Index of British Shells, pl. xiv., fig. II.