SPECIES OK PIS1DIUM. 113 



curving outwarls, acuminate; ridt/es fairly sharp, 

 distal very steep at first, then gradual, umbonal fairly 

 steep. 



#. prominent, obtusely triangular, apex directed back- 

 wards and towards uinbo, base continuous with that of 

 a. n. 



4- laraelliform, sharp, faintly arcuate, running diagonally 

 at a rather low angle across the hinge-plate to near its 

 inner margin. 



p. u. about \ the length of the hinge-line, rather narrow, 

 strong, curving with the hinge-line and inclined out- 

 wards ; Ixise strong and swollen ; apex distal side of 

 centre, distally directed, very prominent, acuminate ; 

 ridges rounded, umbonal descending fairly gradually, 

 distal very steeply, almost vertically at first, then 

 gradually. 



Dimensions. None are given by Clcssin ; Westerlund, however 

 (Faun. Paliiarct. vii, p. L'5) gives, Long. 4, Alt. ,'3, Crass. 2-5 mm. 

 Specimens from Souset (Norway) sent by Dr. Johansen are some- 

 Avhat smaller and measure: 35x 3x2-4 mm. The Perthshire 

 examples received from Dr. Laidlaw attain 4-4 x 4'4 x 3-3 and 

 those from Mr. Jackson from Hawes Water 3'7ox3'4x2'0 mm. 



Clessin when describing the type founded at the same time a 

 " var. transversah" on i)iiite immaterial superficial features. 



Although externally this species, and especially the Norwegian 

 types, at first sight recalls P. obtusale, its stronger striation, and 

 the angle at junction of the dorsal and posterior margin serve to 

 distinguish it, whilst its very well-marked hinge characters readily 

 differentiate it from that and all the other species. 



Specimens from Sonset, Norway, named by Clessin are shown 

 on PI. XXVIII, figs. 17 rt-c, 19 a, "b, & 22 a-j. The Irish specimens 

 closely resemble them but are on the whole larger (PI. XXV III, 

 f. 18 a, 6, &25c,d). 



The species varies somewhat in outline even in a given locality. 

 Thus those from Lough Aguse, Co. Fermanagh, range from tho 

 normal orbicular (PI. XXVIII, f. 25 c & d) through the obliquely 

 orbicular (f. 25 e &f) to oval (f. 2-"> a & b}. The obliquely orbi- 

 cular form also occurs at Lochan a' Chair, Perthshire (PI. XXVIII, 

 f. 13 <i, b). Similar variation is also noticeable in fossil examples 

 from the Holocene at Gayficld, Edinburgh (PL XXVIII, 

 f. 20 a-d, & 24 a-d). In these and other species of Pisidlum 

 from the same deposit, as already noted, the nmboncs are excep- 

 tionally developed. Occasionally the external sculpturing is so 

 marked as to give rise to a superficial resemblance to P. pulchfllum 

 (PI. XXIII, f. 2). 



