46 CATALOGUE OF 



near distal end, very prominent, and distally directed ; 

 ridges slightly rounded, umbonal descending fairly 

 rapidly, with slight convexity, distal exceedingly 

 steep. 



Dimensions. As given by Jenyns, translated into millimetres : 

 Long. 3*5, Alt. 3, Crass. 2 mm. A specimen from Mottram St. 

 Andrews (Cheshire) in Mr. Oldham's Collection attains 4 x 3-4 x 2-2, 

 whilst another in the same collection from Dunster (Somerset) is 

 3-4x3x2-3. 



Jenyns laid particular emphasis on the 3-5 deep cut strite 

 surrounding the nepionic shell, which he states were more or less 

 obvious in every specimen he had seen. These striae are certainly 

 often a feature in the species but cannot be held characteristic of 

 it since they are far from uncommon in P. pitsilhim, in which, 

 however, they are both coarser and placed wider apart.* 



The specimens sent by Jenyns to Hanley, which are in the 

 British Museum collection (1907.12.30 : 519-519) are thinner tl.an 

 the typical form. 



Its nearest ally is P. personatum, from which it may be distin- 

 guished by its usually less rounded form, by the absence of the 

 peculiar callus in the hinge characterizing that species, by the 

 greater flexure of the cardinal (3) of the right valve, and the 

 more oblique set of the cardinals 2 and 4 of the left valve. 



From small forms of P. casertanum, it can be separated by 

 the extreme anterior projection of the valves lying nearer to a lino 

 drawn through the adductor scars., and by its somewhat more 

 regular striation ; while the hinge is not so pronouncedly flexed, the 

 apices of the lateral teeth of the right valve are nearer the umbones, 

 the cardinal teeth of the left valve are more parallel to the hinge- 

 line and 4 is not twisted. 



Contrasted with P. pusilb'.m it is less aaquilateral, the umbones 

 are generally less prominent and the anterior dorsal margin 

 usually slopes more sharply away from the umbo ; the hinge-plate 

 is broader, especially under the umbo, and more flexed ; the apices 

 of the lateral teeth are nearer to the umbo, and the outer laterals 

 (a. in. & p. in.) of the right valve are proportionately much 

 shorter; the cardinal 3 is flexed instead of nearly straight; in 

 the left valve the cardinals are stronger and not so parallel to 

 the hinge-line. 



The specimen figured (PL XIX, f. 11) from Lough Fern, Co. 

 Donegal, may be taken as an example of the typical form. 



Several similar ones may be noted among the fossil forms 

 (PI. XIX, f- 25 a, b ; 31 a, b ; 34 d, ,/). 



Very rounded individuals are shown from Swinton, Lancashire 

 (PI. XIX, f. 1 a, b & 17 , 6), from Lochmaben, Dumfrie.-shiie 



* Tliis feature misled Jeffreys, for a set of P. pusillum in the Norman 

 Collection at, the British Museum marked " P. nitidum examined by Jeffreys " 

 [1911. 10.26 : 7826-45] exhibits this feature, and in other respects also tallies 

 closely with Jeffreys' description of P. nitidum in his "British Conchology." 



