SPECIES OF IMSIUII'M. 53 



5. Pisidium personatxim, Malm. 

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



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



Saiuh. JIandl., Hft. iii, p. 107, %s. 

 1850. ? Pisidium nitidum, \Kt.splendens (BaudonMS.) : Moquin-Tandon, 



Hist. Nat. Moll. France, ii, p. 586; Baudon, Mem. Soc. Acad. 



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



specimens in Norman Coll., B.M. 98. 5. 20 ; 22246-49.] 

 1874 Pisidium fotsarinum, var. curium : Clessiii in Ku'ster, Syst. Conch. - 



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



Coll., B.M. 98. 5. 20 : 22259-60.J 

 1890. Pisidinm fontinale, Tar. curium, Cless. : Westerlund, Fauna 



palaaret. Region, vii, p. 26. 



Pisidium obtusale, var. pecsonatum, Malm : id,, torn, cit., p. 35. 



189J. Pisidium clessi?ii, n. sp. [pa?-s] : Surbeck, Rev. Suisse Zool. 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 pr*dita, nitidissima ; lutescens, 

 margine excepto, lutescenti-cinerea, sed sorde ferruginea 

 omnino obtecta ; natibus convexis, umbonibus subprominulis. 

 Sipho perbrovis, validus et ad extrcmitatem valde con- 

 strictus." 



To this may now be appended the following details concerning 

 the hinge : 



Hinge (PI. I, f. 7 ; III, f. 5) about 1 the length of the shell, 

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

 flexure in right valve. 



K.V. a. /. 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 |- 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. 



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

 margin, slightly arcuate and somewhat outwardly 

 inclined, top crcnulate and highest in the centre. 



