P. parvulum, fossil in Ireland. 347 
P. parvulum, the balance, again, being P. supinwm, although 
the adult form has not yet been recorded from there *, 
None of the specimens from the Pleistocene deposits of the 
Thames received as P. parvulum from either Mr. Phillips or 
Mr. Stelfox could at all be made to agree with true examples 
of the species or the diagnoses in the paper. Since stray 
valves of P. lilljeborgit have been found in these deposits, 
there is no inherent improbability in the occurrence of its 
northern congener P. parvulum; but though the writer has 
again carefully searched his abundant material, no example 
can so far be found. . 
Shells supposed to be living P. parvulum from the Grand 
Junction Canal in Hertfordshire, shown me by Mr. C. Old- 
ham, are merely fry of P. supinum. 
The characters on which Mr. Phillips relies for differen- 
tiating P. parvulum from the closely similar young individuals 
of P. supinum cannot altogether be accepted as valid. Thus 
he says :—‘“‘ In the left valve the base of the lower cardinal 
tooth (¢ 2) is not continuous 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.” It is so figured by Mr. Stelfox in 
the accompanying plate, and when first placed under the 
microscope and superficially examined, the specimen figured 
appears to bear out the statement. When, however, it is 
earefully examined and tested by varying the incidence of the 
light, this conclusion is dispelled. ‘The true base of the 
cardinal tooth proves continuous along the edge of the hinge- 
plate with the lateral, as the well-known morphology of the 
Pelecypod hinge would postulate, whilst the anterior margin 
of the tooth it is that tails off partly in the manner, but not 
wholly in the degree described. Specimens, moreover, in my 
collection are not wanting in which this margin has also 
preserved its continuity with the lateral tooth, The same 
feature is observable to a greater or less degree in other 
species of the genus in which the cardinal teeth are set more 
or less at an angle to the hinge-line. This, then, is not a 
specific character. Nor is the depth of the ligament-pit, 
which an examination of extensive series shows to vary with 
the individual and with age; some of my P. parvulum have 
a very well-marked ligament-pit, with a ridge on the ventral 
side. The degree of application of the dorsal marginal teeth 
in the right valve to the shell-margin is likewise not a 
reliable criterion; adult and unmistakable examples of 
* Some very thickened forms of P. casertanwm do occur in the deposit 
that much resemble P. supinwm, and may have led to the latter being 
overlooked. 
