1915- ^O- 12. THE STROPHOMENIDAE OF THE KRISTIANIA REGION. IO3 



Of the genera met with in the Kristiania region, Triplecia and Streptis 

 have been included in one subfamily, Tripleciinac, and Christiania in 

 another, Davidsoniinac. In m}' opinion still other groups ought to be 

 kept separate, e. g. the genus Plectambonitcs, at any rate if the two main 

 groups, the subfamilies Rafinesquininae and OrtJiotetinae are maintained. 

 In m}- opinion a Rafinesquina and a Strophomena are as much or more 

 related as a Rafinesquina and a Plectamboniles. Yet, a certain settlement 

 of this question cannot be reached till the true relation between the 

 Rafinesquinas and Strophomenas is better known. To this question for 

 which investigations of the oldest Strophomenids and their lines of descent 

 are especially important, I hope to be able to contribute in my study of 

 the Russian material of Strophontenidae. 



Occurrence of the Strophomenids in different 

 Kinds of Rock. 



In order to give an answer to this question I have prepared the 

 following table, based on the material that has been studied in this work. 

 Here all the above described forms, of which the inclosing matrix is 

 known, are found, with reference to the type of sediment in which they 

 occur. Those marked in the first column have been found only in lime- 

 stone (compact or as thin la3'ers or nodules, pure, arenaceous or some- 

 what argillaceous), those in the second column only in shale, those in the 

 third in both limestone and shale. 



The table thus gives the result, that of 70 forms 49 have been found 

 in hmestone onl}', 9 in shale onl}^ and 12 in both kinds of sediment. If 

 we do not consider those cases where the fossil is found as a single 

 specimen or as a few specimens from one layer and one locality, in a 

 total of 19 forms, we obtain the figures 36 in limestone, 3 in shale and 

 12 in both. As might be expected the result is that limestones is exeedingly 

 much richer in Strophomenids than the shales. As will be understood from 

 the stratigraphie table p. 10 — 11, both limestone and shale are found in 

 important quantities in every one of the stages 3 — 9, except in stage 5 

 (to some degree also in 9) where limestones is by far the prédominent 

 type of sediment. If we therefore remove the 1 1 species that are exclusive!}' 

 known from 5 and therefore might be expected to occur only in limestone 

 we obtain the figures 25, 3 and 12, of species occurring in stages with 

 much of both kinds of sediments. 



