I04 The Irish Naturalist. July, 
with that of the anterior lateral (a. II.) but runs at an 
angle across the top of it and tapers off to near the shell- 
margin. In the right valve the outer laterals (a. III. and 
p. III.) are much curved at both ends towards the interior 
of the shell and are separated from the shell-margin by a 
wide, deep groove. 
This species has, in England, been confused with and 
mistaken for the young of P. siipimim, but the two ma}' 
be distinguished b}^ the following points of difference : — 
(a.) The ligament pit of even the youngest P. sitpinum 
seen possesses a strongly-marked ridge along 
its ventral edge. The pit of P. parvuluni is 
only a faint hollow without a distinct ridge. 
(/;.) The cardinal tooth in the right valve of P. supinum 
is thicker and more curved than that of P. 
parvulum. 
{c.) In P. supimun the outside lateral teeth of the 
right valve lie very close to and almost parallel 
to the shell-margin ; in old specimens of the 
very thickened form they are almost completely 
merged into the shell wall. In P. parvulum 
the}/ are distinctly separated from the shell- 
margin and are much curved inwardly, especially 
at their lower extremities. 
P. parvulum was first detected last January among 
^shells obtained in sand brought in 1913 from a deposit in 
the Suir near Fiddown ; samples of material taken from 
the same locahty at various times in subsequent years have 
yielded additional specimens. The very perfect pair shown 
on Plate II., figs. 7-10, were taken by scoop with living 
examples of other species in the Suir near Clonniel in 
April, 1916. It is abundant in the Barrow deposits above 
and below New Ross and near Graiguenamanagh. 
In sand obtained this year from the Shannon near Plassy 
about two miles above Limerick it is fairly plentiful. 
It also occurs as a fossil in the Pleistocene deposits of 
the Thames valley, as Mr. Stelfox discovered numerous 
specimens of a very tliickened form mixed with young 
