SPECIES OF 1'ISIDIUM. 55 



whilst the cardinal teeth in hoth valves are not so sharp and 

 straight. 



From P. casertanum, even small, lake forms, it is distinguished 

 by its more globose form, its straighter cardinals, Avhich are more 

 strictly parallel with the hinge-line; and by the proportionately 

 larger anterior lateral, a. ///., which lies against the shell-wall 

 instead of standing out by itself. 



Above all it is usually immediately separable from all by the 

 hall-mark of the species, the peculiar callus that lies in either 

 valve between the umbonal ends of the posterior lateral teeth 

 and the ligament-pit. 



As typical forms we would select those from Fitton Hall, 

 Oldham (PI. XX, f. 1), Bassenthwaite (PI. XX, f. 2 & 20 a, b) and 

 from the Holocene of the Kennet Valley (PI. XX, f. 26 e & g). 

 Extremely oval forms are met with from Lough Fern, Co. 

 Donegal (PI. XX, f. 3d & 33 a, b) and from the Holocene of the 

 Kennet Valley (PI. XX, f. 26 h). 



Forms that approach the lake-form of P. casertanum in external 

 shape are shown from Lancaster (PI. XX, f. 16) and Fair Head, 

 Co. Antrim (PI. XX, f. 17). As an example of the normal inflation 

 of the species may be taken that of a specimen from Barnes 

 Common (PI. XX, f. 4) ; whilst Lancaster yields a flattened example 

 (PL XX, f. 5), and on the other hand extreme inflation is ex- 

 hibited by specimens from Colchester (PI. XX, f. 7) and Ziegenhals, 

 Silesia (PI. XX, f. 8). 



DISTRIBUTION. 

 RECENT. 



ENGLAND. 1 

 LONDON. H. 



Barnes Common [B. B. W.] [W. M. W.] ; Hampsfcead [J. E. C.] ; 



Putney [W. M. W.] ; Tottenham (11. Lea) [W. M. W.]. 

 CHESHIRE. CH. 



Baguley [C. 0.] ; Bramhall [C. H. M.] ; Mobberley [C. 0.] ; 



Mouldsworth [A. S. K.] ; Northenden [C. 0.]. 

 CUMBERLAND. CU. 



Bassenthwaite [W. J. F.] ; Rockcliffe [J. L.]. 

 DEVONSHIRE, NORTH. ND. 



Braunton [J. L.] ; Croyde [J. L.] ; 3Iorthoe district [J. L.]. 

 DURHAM. DM. 



Butterwick [B.M. 1911.10.26: 7949-61.]; Elwick [B.M. 



1911.10.26: 7873-78]. 

 ESSEX, NORTH. NE. 



Birch [W. M. W.] ; Colchester (?; [W. M. W.] ; West Bergholt 



[B.M. 47-11-19: 163-177]. 

 ESSEX, SOUTH. SE. 



Warley [W. M. W.]. 



