SPECIES OF FJSIDIUM. 53 



5. Pisidium personatum, Malm. 

 [Pis. I, f. 7; III, f. 5; XX.] 



1855. Pisidium personatum, n. sp. : Malm, Gotheborgs K. Vet. & Vitt. 



Samh. Handl., Hft. iii, p. 107, figs. 

 185G. ? Pisidium nitidtim, v&r.splendens (Baudon MS.) : Moquin-Tandon, 



Hist. Nat. Moll. France, ii, p. 586; Baudon, Me* in. Soc. Acad. 



Oise, Beauvais, iii, 1857, pp.335 & 337, pi. i, figs. B.B. [Cf. 



specimens in Norman Coll., B.M. 98. 5. 20 ; 22240-49.] 

 1874 Pisidium fwuarmum, var. curium : Clessin in KUster, Syst. Conch. - 



Cab., ix, abth. 3, Cycladeen, p. 34. \_Cf. specimens in Norman 



Coll., B.M. 98.5.20: 22259-60.J 

 1890. Pisidium fontinale, Tar. curium, Cless. : Westerlimd, Fauna 



palaarct. Region, vii, p, 26. 



Pisidium obtusale, var. pei'sonaium, Malm : id,, torn. f:it., p. 35. 

 189J. Pisidium clessini, n. sp. [pars] : Surbeck, Rev. Suisse Zoi:l. vi, 



p. 482, pi. xii, f. 6-15. 



The detection in 1908 (191, p. 124) of this somewhat variable 

 species in the British Isles has done much to clarify the con- 

 ception of several others, such as P. casertanum, P. nitidum, and 

 P.pusillum, with which it had generally been mixed. 



Malm's diagnosis (106, p. 107) is : 



" C. orbiculato-ovalis, compressiuscula, tenuissime striata, 

 sulcisque 1-5 concentricis pruedita, nitidissima; lutescens, 

 margino excepto, lutescenti-cinerea, sed sorde ferruginca 

 omnino obtecta ; natibus convexis, umbonibus subprominulis. 

 Sipho perbro-vis, validus et ad extremitatem valde con- 

 st rictus." 



To this may now be appended the following details concerning 

 the hinge : 



Hinge (PI. I, f. 7 ; III, f. 5) about | the length of the shell, 

 wide, projecting well inwards, very strong, arcuate, with double 

 flexure in right valve. 



K.V. a. i. a little more than ^ the length of the hinge-line, very 

 stout and strong, curving inwards ; base very strong 

 and swollen ; apex on distal side of centre, prominent, 

 somewhat distally directed, pointed obtuse ; ridges 

 rounded, distal descending very steeply at first-, then 

 gradually, umbonal sloping somewhat less steeply and 

 descending to a lower level. 



a. in. nearly J the length of a. /., narrow, strong, apposed 

 to the shell-margin ; apex distal side of centre, 

 prominent, blunt obtuse ; ridges rounded and sloping 

 about equally. 



8. lamelliform, very prominent, lying close to the shell- 

 margin, slightly arcuate and somewhat outwardly 

 inclined, top crenulate and highest in the centre. 



