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SPH^BIUM NITIDUM, CL., A SIBERIAN FRESHWATER MUSSEL, 



IN SWEDEN. 



By Dr. Nils Hj. Odhnee. 



(Communicated by B. B. Woodward, F.L.S.) 



Read I2th November, 1920. 



On a revision of the Swedish freshwater mussels of the family 

 Sphaeriidae it appeared that the genus Sphcerium (excl. Musculium) 

 is represented in the Swedish fauna by two species, S. corneum, L. 

 (comprising also S. draparnaldi, ovale, and mamillanum) and the 

 Siberian S. nitidum, Clessin (Martini and Chemnitz, Conch. Cal., 

 Kiister's new ed., 1877 ; of. also Westerlund, " Sihiriens Land-och 

 Sbtvatten-Mollusker,'" K.Sv. Vet. Akad., Handl. 11, 1877). The 

 latter species originally recorded from Jenissei, proves to be well 

 separated from S. corneum not only in its shell (cardinals are straight, 

 not curved as in corneum, and cardinal 4 covers only the rear half 

 of 2, the hinge-plate is very narrow, the umbones usually inflated), 

 but also in its anatomy. The best distinguishing character is ofiered 

 by the nephridium, which lies as a paired mass between pericardium 

 and posterior adductor, and is very easy to examine. Seen from above 

 the dorsal parts of this organ have a difierent aspect in S. corneum 

 and S. nitidum. In the former the dorsal surface of each nephridium 

 has a U-shaped appearance, and both legs of the loop are 

 separated by an interstice occupied by a small protruding part from 

 the interior of the nephridium (the apical flexure of the inner 

 or pericardial tube). In S. nitidum the two legs are entirely closed 

 together, so that the inner parts of the nephridium are totally covered 

 and do not protrude dorsally . Further, the dimensions of nephridium 

 in relation to pericardium and posterior adductor are different 

 from those of S. corneum, where each nephridium, seen from above, 

 is short and broad, its length only slightly surpassing that of the 

 adductor and that of the pericardium ; its breadth is greater than half 

 its length. These characters were constant in all forms of S. corneum 

 examined. In S. nitidum the nephridium is twice as long as broad, 

 and occupies twice the length of the adductor and twice that of the 

 pericardium. 



That the Swedish specimens are identical with the Siberian 

 form was proved by a comparison with specimens from the original 

 locality ; the characters of the shells as well as of the soft parts are 

 similar in both cases. 



This interesting species has been found only in northern Sweden, 

 north Lappland (whence it was mentioned by me in 1908 as 

 S. corneum and S. ovale), south Lappland, and Jamtland (Coll. in 

 Swed. State Mus.). It is entirely wanting in south Sweden, even 

 in the deep and cool lakes where arctic relics are recorded 

 {e.g. Vattern), and it therefore seems likely that S. nitidum is a rather 

 late immigrant to the Swedish fauna, and that it has a direct eastern 

 origin. 



