SPECIE? OF r:srrm:.\T. 109 



4. lamelliform, sharp, curving backwards round ~ to near 



the inner margin of the hinge-plate. 



p. it. not quite \ the length of the hinge-line, very narrow, 

 fairly strong, straight; base fairly strong; aper at 

 distal end, distally directed, prominent, acuminate ; 

 ridf/es sharp, umbonal descending gradually, distal 

 very steeply at first, then gradually. 



Dimensions. Moller gives simply " Diam. 2'" " which is equi- 

 valent, to 4'2 mm. Morch gives Long. 4, Alt. 3*5 mm., and Clessin 

 Long. 5, Alt. 4, Crass. 3 mm. An example sent from Finmarken 

 (Norway) by Dr. Johansen is 4'8 x 4 x 4 mm. The English 

 specimens are small, the largest- taken by Mr. Carter in Burwell 

 Wood (Lincolnshire) is 3'8 x 3-3 x 2*8 mm. 



The 4 or 5 more strongly marked lines of growth due to 

 coloration noted in the preceding descriptions, though frequent in 

 the species, are not peculiar to it. 



The species has nothing in common with Pisidium pusiilum of 

 Turton, nor of Jeffreys as alleged by Clessin. 



From even small rounded forms of the still-water variety of 

 P. casertanum, or from P. nitidum, it may be distinguished by being 

 yet rounder and having the hinge lighter and slightly more arcuate : 

 its outer lateral teeth (a. ///.. p. ///.) are proportionately longer, 

 and all the other laterals have their apices nearer the umbo ; in the 

 cardinals, 3 is more sharply flexed and thinner, 2 is more back- 

 wardly directed, whilst 4 is more curved and not so diagonally 

 placer]. 



Compared with the smoother forms of P. lilljebonjii it is less 

 orbicular, less regularly striate, and lacks the angularity at the 

 junction of the dorsal with the posterior margin; the hinge is 

 longer and less arcuate, the inner and outer lateral teeth of the 

 right valve are not so equal in size and do not converge towards 

 the umbo ; whilst in the cardinals of the left valve 2 is less 

 strong and more backwardly directed and 4 does n t extend so far 

 backward. 



Taking the specimens from the type locality Greenland as being 

 the most normal (PI. XXVIII, f. 6, 7 & 10), it will be seen th;it 

 the specimens from Bjerldng, Norway (PI. XXVIII, f . 3 & 15), 

 and from Finmarken, Norway (PI. XXVIII, f. 16), both of which 

 are examples of Clessin's P. ylobulare, cannot be separated from 

 them, though the last-named are slightly more oval. 



Of the British representatives those from Grisel Bottom, Lin- 

 colnshire (PI. XXVIII, f. 5, 8 a, 6, & II a, 6), come neanst to the 

 type, but are slightly less tumid, whilst the specimens from Abbey 

 Holme, Cumberland (PI. XX VI 1 1, f . 2 & 4), are both smaller and 

 more oval. 



