OLAF HOLTEDAHL. M.-N. Kl. 



As to the appearence of the denticulation of the cardinal areas, it 

 does not seem to be bound to any particular geologic time, in other words 

 the Rafinesquinas and Strophomenas do not disappear and the Stropheo- 

 dontas and Strophonellas not appear all at one time. This is also 

 emphasized b}' Hall and Clarke (see above p. 97) In my opinion this 

 fact simply means that the denticulation appears at different times 

 in different evolutionary lines. While w^e meet with denticulation in 

 Amphistropliia Davidsoiii from stage 6 we find smooth cardinal margins 

 in Strophomena antiqiiata still in 8. 



Even if the material described in this paper is too poor to give any 

 quite certain answer to the question of the Stropheodontas being the 

 direct descendants of the Rafinesquinas, the Strophonellas of the Stropho- 

 menas, we at any rate do not meet any fact that contradicts this supposition. 

 The circumstance that very young individuals of Strophonellas are found 

 to have convex ventral, and concave dorsal valves cannot be regarded as 

 a sufficient proof of their being more closely related to Stropheodontas 

 than to Strophomenas, the resupination of the valves may have taken 

 place at a very early stage in the phyllogenetic development. 



Returning to the classification and the nomenclature, however, I 

 think the older and the younger types of what I consider the line 

 Rafinesquina — Stropheodonta, Strophomena — Strophonello , in their typical 

 representatives, (the older from the Ordovicic, the younger from the 

 Devonic) are so distinctl}' different, that the terms mentioned may very 

 well be given generic value. 



We will now briefly mention another question concerning the classi- 

 fication, namely the relation beween normally convex and resupinate 

 forms. A series of facts that were met with during the investigation, 

 strongly point towards the conclusion that the curving of the valves in 

 one or another direction cannot, if not accompanied by other decisive 

 characters, be considered as a systematic character of great importance. 



We will take some examples. One of the oldest forms met with is 

 Strophomena (?) Nefedjewi having a slightly concave ventral, a moderately 

 convex dorsal valve. Nevertheless, as is mentioned elsewhere, its nearest 

 known relatives are found in a group of Strophomenids with normally 

 curved shells, in the genus Plectella. 



In the resupinate Strophomena rigida Barr, we observe a quite pecu- 

 liar, very fine surface ornamentation, the surface in the intervals between 

 the major ribs showing a series of small undulations besides having a 

 lattice-like appearance, caused by crowded, very fine radiating and con- 

 centric lines crossing each other. Exactly the same type of ornamenta- 



