woodward: on some species op pisidium. 215 



Opportunity having lately presented itself to carry out a long- 

 deferred intention to reinvestigate the matter, I applied to 

 Mr. Oldham, who most kindly lent me the whole of his valuable 

 collection of the species, which is specially rich in examples from 

 remote Welsh tarns. To him my thanks are due and hereby tendered, 

 as well as to Mr. Cooper and Mr. Overton, for the chance of inspecting 

 additional gatherings. 



Studied thus in the bulk, a truer appreciation of the species as a 

 whole is possible. The outstanding feature externally, in addition 

 to its globular form, in which it comes nearest to P. obtusale, is the 

 fact that the lines of growth are not only " regular, close, and well 

 marked ^^ (p. 36) ", but that they are present on the nepionic shell. 

 It differs in this respect from both P. nitidum and P. pusillum, in 

 which the nepionic shell is usually smooth, followed by a band of 

 strongly marked, deeply incised lines, a feature noted by Jenyns as 

 characteristic of the former, but overlooked by him in the latter 

 species. In P. Jiihernicum, moveover, there is a suggestion when 

 viewed under the microscope, of faint radial sculpture rarely present 

 in other species. Internally the chief feature is the short hinge-line, 

 just as in P. obtusale, and, in fact, all orbicular bivalves. This 

 shortness of the hinge-line persists in most of its form mutations, 

 although if the determination and drawings be correct, it is somewhat 

 departed from in the specimens from Gortymadden figured by 

 Phillips and Stelfox ^^ (pi. i, f. 7 and 8). In addition the points of the 

 lateral teeth, when viewed sideways, stand up more sharply, as shown 

 in the " Catalogue " than they do in other species of the genus. 

 Diagrammatically expressed they are A A rather than 'N •^ . 



A thorough examination was made of all the samples in the 

 Kennard- Woodward collection, with the result that a few Irish ' 

 representati^^es were detected as misplaced, namely : under obtusale, 

 examples from Lough Acapple (Donegal) ; Tully Lough 

 (Fermanagh) ; and Lough Nalackan, Brandon Mt. (Kerry) ; Avhilst 

 a tube of " pusillum " from Lough Nafeola (Fermanagh) proved 

 divisible into Jiihernicum and nitidum. An equally careful research 

 in the collections at the British Museum (Natural History) yielded 

 no instance of any misidentified Mbernicum. 



List of occurrences so far seen and determined by the writer : — 



BECENT. 



EnC4LAND. 



Bedfordshire. — BD. 



R. Ouse, Bromham [C. 0.]. 

 Buckinghamshire. — BX. 



R. Thames, Denham Lock [J. E. C.]. 



R. Colne, Iver [J. E. C.]. 



Wendover Canal, Halton [C. 0.]. 

 Cambridgeshire. — CB, 



R. Ouse, Ely [C. 0.]. 



