50 



OLAF HOLTEDAHL. M.-N. Kl. 



forms. They differ, however, distinctly in size, the said species being 

 decidedly smaller than S. Brøggeri. In interior characters the two forms 

 are, as will be understood, distinctly different. 



An interesting fact concerning the outer form of this species is the 

 following: among the more than 30 ventral or dorsal valves present in 

 the same rock specimens, I found a specimen, that shows a normal — 

 not reversed — geniculation, the ventral valve being convex, the dorsal 

 concave. The specimen is depicted in pi. VII, fig. 4. As will be seen 

 from the illustration, the other external characters exactly correspond: 

 Size, outline and surface ornamentation. As only this single specimen is 

 present I have not tried to get at the interior with acid, which scarcely 

 in this case, when both valves are present in natural position, would 

 give any result. Thus the only proof as to which is the ventral and 

 which the dorsal valve is the outer form, but this, however, is quite 

 sufficient, one of the valves showing a distinct convexity in the umbonal 

 region (indicating the ventral valve), the other being flat or rather faintly 

 concave in the corresponding portion of the shell. 



This fact then evidently shows that the character of resupination 

 cannot be a very stable one in this species. We know of similar con- 

 ditions from American Siluric Strophonclla forms, but here the convexity 

 of the ventral, and concavity of the dorsal valve are found only in young 

 stages, indicating such »normally» curved forms as the older type. 



Occurrence. 4 b, Bratterud (trans. 4 b « — /S), and Frog, Ringerike. 



Strophomena, (?) cf. Asmusi Verneuil. 



A small portion of the posterior part of what certainly must be the 

 ventral valve of a nearly flat Strophomenid is all that exists of this form 

 amongst the material. It has been a fairl}^ large form, with a faint con- 

 cavity in the valve, just like that oï Strophonicim Asmusi. On the strongly 

 worn surface are seen a number of equal radiating ribs, at a distance of 

 generally less than i mm. in the anterior portion of the fragment. 

 Although nothing can be distinctly seen, there seem in some places to be 

 indications of much finer ribs between the others. Even if the general 

 form of this specimen shows a likeness to S. AsDinsi, it can easily be 

 differentiated by the major ribs (if they are such), coming more close to 

 each other. 



Occurrence. 3c /^, Slemmestad, SW of Kristiania. 



