SPECIES OF PISIDIL'M. 113 



curving outwar Is, acuminate ; rtdijes fairly sharp, 

 distal very steep at first, then gradual, umbonal fairly 

 steep. 



2. prominent, obtusely triangular, apex directed back- 

 wards and towards umbo, base continuous with that of 

 a. if. 



4. laraelliform, sharp, faintly arcuate, running diagonally 

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

 inner margin. 



p. n. about | the length of the hinge-line, rather narrow, 

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

 wards ; hase strong and swollen ; apex distal side of 

 centre, distally directed, very prominent, acuminate ; 

 ridge* rounded, umbonal descending fairly gradually, 

 distal very steeply, almost vertically at first, then 

 gradually. 



Dimensions. None are given by Clessin ; Westerlund, however 

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

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

 what smaller and measure: 35x3x2'4 mm. The Perthshire 

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

 those from Mr. Jackson from Hawes Water 3*75 x 3-4 x 2-6 mm. 



Clessin when describing the type founded at the same time a 

 " vur. transversale " on quite 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 

 (list inguish 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 -c, 19 , b, & 22 a-/. The Irish specimens 

 cloudy resemble them but are on the whole larger (PL XXVIII, 

 f. 18 a, b,&2oc&d). 



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

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

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

 orbicular (f. 25 e &/) to oval (f. 2o a & b). The obliquely orbi- 

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

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

 from the Holocene at Gayfield, Edinburgh (PI. XXVIII, 

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

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

 tionally developed. Occasionally the external sculpturing is so 

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

 (PI. XXIII, f. 2). 



