588 Karl A. Grönwall. 



not lead to any addition to what Wiman has already mentioned 

 with regard to the dorsal valve. 



This species is one of the most common in the Spirifer lime- 

 stone of Spitzbergen, and also occurs in the same horizon in Beeren 

 Eiland. In Russia it is specially general in the Timan district, be- 

 ginning there in the Omphalotrochus horizon, and having its chief 

 occurrence in the Schwagerina horizon. In the Ural district again 

 it is rare, being found there in the Cora horizon and also in dolo- 

 mites belonging to the Arta stage. 



I have not been able to find any younger specimens among the 

 material from Spitzbergen in the State Museum at Stockholm that 

 has been determined by Wiman, but among fragments which have 

 been characterized by him as "undeterminable Productidæ" I suc- 

 ceeded in discovering some younger specimens of Pr. timanicus, which 

 agreed perfectly with those from North-east Greenland. In the col- 

 lection from Spitzbergen of the Geological Institute of Uppsala there 

 were found no younger specimens that could throw any light over 

 the question of the structure of the juvenile forms. 



As to the possible occurrence of this species in the Arctic North 

 America the reader may be referred to the detailed unravelling of 

 this problem, that is given below, pp. 602—604, where, too, the litera- 

 ture dealing with the question is also mentioned. 



