KTJNGL. SV. VET. AKADEMIENS HANDLINGAR. BAND 59. NIO 3. 7 



and vice versa. Such a condition has, however, never been observed either as a 

 teratological phenomenon or as a generic acquisition. In the literature I have met 

 with only a few statements of something like inversion in Lamellibranchia. One 

 was observed by Kimakovics (1897) in Asiarte sulcata. He says (p. 80): »Eine von 

 der Sassenbai stammende rechte Klappe hat linksschalige Schlossbildung. i This ob- 

 servation is verified by v. Vest (1899, p. 37), who writes: Dass aber eine Schloss- 

 verwechslung auch bei frei lebenden, d. h. nicht angewachsenen Arten vorkommt, 

 diirfte denn doch zu den selteneren Vorkommnissen gehören. So hat eine von Spitz- 

 bergen (Sassenbay) mitgebrachte rechte Klappe der Asiarte corrugaia Brown ( = serai- 

 sulcata Leach) ein Linksschloss . . . ; nämlich statt des ihr als rechter Klappe zu- 

 kommenden dicken Kardinalzahnes und der beiden seitlichen kleineren Schlosszähne, 

 zeigt obiges Exemplar den Schlossbau der linken Klappe mit zwei gleichen Kardinal- 

 zähnen und einer grossen Zahngrube dazwischen.» The valve is figured by the author 

 in plate I, fig. 9. 



A second case of an assumed inversion has been described by Reynell (1908) 

 in a short notice on Asiarte miitabilis. He reproduces a left valve, »which at once 

 shows the large central cardinal tooth of the right valve instead of two diverging 

 ones with a pit between, which are characteristic of the normal left valve». 1 



Whether the aberrant hinge construction of the valves in question is a rea! 

 inversion may, however, be doubted; it might be due to a mere displacement of the 

 left median cardinal and of the right anterior one somewhat backwards, compensated 

 by a stronger development of the left anterior one and the disappearance of the 

 posterior cardinal tooth. 



That this supposition has something to support it I have found by studying 

 for the purposes of a monograph a large series of shells of the genus Pisidium, be- 

 longing to the collections of the Swedish State Museum. Among these I observed 

 some cases of abnormal dentition representing this case of reversibility. A specimen 

 of Pisidium scholtzi from Lake Enare offered the aspect of two cardinals and one 

 anterior and one posterior lateral in the right valve; in the left valve there were 

 one cardinal and, on each side, one well-developed inner and one rudimentary outer 

 lateral. The hinge was, consequently, inverted with respect to normal conditions. 



In another case a specimen of P. steenbuchi from Claushavn, Greenland, was 



examined. It too showed the inverse hinge construction, though it occurred together 



with many shells of normal dentition. In this specimen, preserved in alcohol, the 



animal was still retained, and, further, had young ones in its gills (2 on each side). 



Two of them were examined in respect to the hinge, that was found to be normal; 



they showed in the left valve tooth 2 (Bernard's denomination, ef. Woodward 



1913); behind this followed tooth 4 and between them there was space for tooth 3 



of the right valve. Both young ones thus had a normal hinge, which proves that the 



inversion of the mother was not hereditary. 



1 liesides the statements quoted above. Fischer (1880) described a »monstrosité sénestre d'une Telline», 

 the posterior end of which was bent to the left instead of to the right: the hinge was not examined, and the 

 monstrosity was therefore of a quite unknown nature. — A similar abnormity was observed by Ltnge (1909) 

 in TeUina semitorta. 



