As a rule this species is recognizable at the first glance owing 

 to its quadrangular shape (PI. XXIX, f. 16 a, 23 & 27). In the 

 few instances in which it assumes a rounded exterior and 

 approaches other species in form, it is at once known by the very 

 narrow and fairly straight hinge-line and the slender cardinals 

 which run in the direction of the hinge-line and in the left valve 

 are parallel to each other. 



PI. XXIX, f. 22 shows one of the more rounded forms similar 

 to Jenyns' Pisidium pulchellum, var. S. 



Fig. 8 on the same plate shows an example whose inflation is 

 a little in excess of the normal, fig. 9 one of maximum inflation 

 and fig. 11 a flattened form. Figs. 10 & 30 illustrate the 

 peculiar variant of the species occurring in the Holocene at 

 Gayfield, Edinburgh, in which the umbones are strangely pro- 

 duced. 



DISTRIBUTION. 

 si 



01 



Li 



Cl 

 Pisidiwn m iliu m . 



(Recent occurrences are shown in W: fossil occurrences by a line 

 below the symbols. For the explanation of the symbols see p. 20.) 



