SPECIES OF PTSIDIUJf. 107 



posterior curving round 2 and terminating close and 

 almost at right angles to the inner margin of the 

 hinge-plate. 



p. ii. about \ the length of the hinge-line, stout, erect, 

 straight; base strong and swollen ; apex nearer distal 

 end, somewhat distally directed, very prominent, sub- 

 acuminate ; ridges sharp, descending about equally, at 

 first steeply, then gradually. 



Dimensions. Clessin's measurements are: Long. 2, Alt. 1*5, 

 Crass. 1'2 mm. Danish specimens received from Dr. Johansen are 

 only 1/5 X 1*4 X 1*4 mm. 



Clessin has also named a variety (179, p. 553) on account of its 

 greater size and solidity. 



The species comes near to P. siqnnum in form, but is squarish 

 instead of triangular. 



The type specimens came from the Blekinge, and the variety from 

 Ronneby in Sweden : the Danish specimens are from Fursoen : it 

 has been received under the name of " P. aliena, Mts." from Lake 

 Baikal. 



13. Pisidium steenbuchii (Moller). 

 [Pis. II, f. 7 ; IV, f. 1 ; XXVIII, f. 2-8, 10, 11, 14-16.] 



1842. Cyclas steenbuc.hu, nob. : Moller, Index Moll. Grcenlandise, p. 20 ; 



tV/., Kroyer, Naturhist. Tidsskr. iv, 1842, p. 93. 

 1851. Pisidium fontinalc, var. ? : MiddendorfF, Rrise Siberiens, ii, till. 1, 



p. 402. 

 1854. Sph&rium steenbuchii, Moller : Deshayes, Cat. Conch. Brit. Mus. 



p. 264. 

 1857. Pisidium steenbuchii, Moller: Morcli in Kink, Grimland, &c. ii, 



till. 4, p. 91 ; id. (reprint) Fortegnel. Gronl. Bloddyr, p. 19. 

 ]869. Pisidium pusi Hum [pars] : Jeffreys, Brit. Conch, v, p. 150. 



1873. Pisidium ylobidare, Cless. i Clessin in Westerlund, Fauna Moll. 



Svec., Norv. et Daiiire, p. 532. 



1874. Pisidium steenbuchii, Morch hie] : Clessin in Kiister,Syst. Conch.- 



Cab. ix, abth. 3, Cycladeen, p. 35, pi. iii, f. 21, p. 22. 

 1886. Pisidinm hoyeri, D. sp. : Clessin in Esnaark & Iloyer, Malakozool. 



Blatt., N.F. viii,p. 120. 

 1909. Pisidium korotnevi, n. sp. : Lindholm in Korotneff, AVissensch. 



Ergebn. Zool. Exped. Baikal-See, iv, p. 85, pi. ii, f. 47, p. 48. 



The species is not common in this country, cither recent or 

 fossil, and its first record from Perthshire was made in error on an 

 abnormal example of P. lilljtbvrgii (193, p. 5). It is stated by 

 Mb'rch to be rare in Greenland, whence it was first described by 

 Holler (115, p. 20) under the genus Cydas as : 



" Testa inacquilatera, subtrigona, antice obtusa, ventricosa, 

 lacvi, cinereo-lutescente ; umbonibus prominentibus ; sulcis 

 increment! 4-5." 



