ODHNEK : ON I'isiriiUM In riiF, swkdisH state MUSRUM. 221 



dorsal side, an entire lobe only slightly concave in front ; this lobe, 

 in other Pisidia, is divided by a fissure. 



Thus P. torquatiim proves distinct not only in conchological but 

 also in anatomical characters. Its umbonal appendicuk"e, as Stelfox 

 has pointed out, are one of its chief external marks, which it shares 

 with P. hensloivannm. In the last-named species, however, the 

 appendiculcC may occasionally be ill-defined or absent, and the same 

 may perhaps occur in P. torquatmn ; is it possible that P. tenuiline- 

 ahtmSteUox may be an analogous inappendiculate and more regularly 

 striate variety ? 



P. torquatum has not been found in Sweden. P. parvuluiii Clessin 

 described from Ronneby, South Sweden, is quite different, as stated 

 by Stelfox ; it is identical with P. hibernicum Westerlund (see below). 



Pisidium clessini Surbeck, 1899. 



{P. torncfise Odhner, 1908 ; P. pusilluiii Woodward, 191 3 



{pars) lion Jenyns. 



The chief organization of P. torquatum is common to the present 

 species ; thus there is only one gill on each side, only one siphon, 

 and a similar nephridium. The shell characters are, however, quite 

 different, and the very weak teeth, the long distance between car- 

 dinals and laterals, as well as the shape of the rather fragile shell, 

 recall P. nitidwii. The chief difference lies in the shape of the 

 cardinals ; tooth 3 (right valve) is situated in the lowest margin of 

 the hinge-plate, and consists of a thick and straight posterior and a 

 slightly curved anterior half. Teeth 2 and 4 (left valve) are short, 

 and the latter does not cover more than the posterior end of the 

 former. The sculpture consists of weak and irregular concentric 

 lines, gradually vanishing towards the umbones. The porosity of the 

 shell is of a singular nature, as I have already mentioned. P. tornense, 

 which was originally described from Lake Torne Trask in Swedish 

 Lappland, was incorrectly identified by Woodward (1913) as P. 

 pusilliDu Jenyns. The same species also occurs in deep and cold 

 lakes ui Southern Sweden (e.g.. Lake Vattern), and subsequent 

 research has convinced me that it is partially identical with P. clessini 

 Surbeck (which, however, includes P. nitidum) from Lake of Lucerne, 

 whence material was sent me by Prof. Zschokke. The name clessini, 

 consequently, must be considered valid for the present ; probably it 

 will prove synonymous with one of Clessin's deep water Pisidia from 

 the Alpine lakes. 



This interesting glacial species has not yet been recorded from other 

 localities. Mr. Stelfox has, however, sent me specimens from two 

 places in the British Islands, viz., "a cold water tarn on Brandon 

 Mountain, at 2,500 feet altitude, Co. Kerry, Ireland" (A. W. Stelfox 



