122 



CATALOGUE OF 



Swollen specimens can be differentiated from forms of P. lill- 

 jeborgii, not only by the above characteristics but externally by 

 their smoother surface and the lack of the angularity of the 

 posterior dorsal margin so typical of the latter. 



That, the species is liable to considerable variation is manifest 

 from the figures (PI. XXX), and is sometimes seen in n. single 

 gathering such as that from Birch (Essex) (PL XXX, f. 1 & 16). 

 One of the more swollen examples from this locality is shown at 

 fig. "2 on the same plate, and compares with that at fig. 5 from 

 T. Rogers' collection named by Jeffreys "P. nitidum, var. globosa." 

 The least tumid form, figs. 4 & 10, came from Guildford; the most 

 oval examples are represented in rigs. 13, 14 & 15, the last named 

 from Lytham being one of the largest met with. The thinnest were 

 those from Lochmaben, tigs. 8 & 18, whilst a quaint abnormality 

 from "Walworth Wood (Londonderry) is portrayed in fig. 7. 



DISTRIBUTION. 



si 

 01 



Cl 



Pisidium obtutale, 



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

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



