OLAF HOLTEDAHL. M.-N. Kl. 



region thé ribs are all of nearly the same size, the differentiation into 

 ribs of different sizes — by intercalation, not bifurcation — becoming 

 more and more expressed towards the latero-frontal margins. 



A series of fine crowded concentric lines can be seen in a good 

 light on some specimens with well-preserved surface characters. Most of 

 ^ the specimens do not show any distinct corrugations, 

 \ only a few lines of growth. In some, however, an 

 indication of wrinkles is seen near the hinge at 

 some distance from the beak, and in one specimen 



/ 



Figure showing the size these few wrinkles are rather well developed. 



of ventral muscular area -,-, • ^u u n u ,. r .1 



^ „ ^ By removmg the shell substance 01 a ventral 



in a specimen of Rafi- -^ 



nesquina mjøsensis. valve i obtained an idea of the form and size of 

 the ventral muscular area, as will be seen from the illustration. It is of 

 rather small size. 



Observations. When knowing only the exterior of this form 

 I thought it identical with Rafinesquitia miwiesotaeiisis N. H. Wln'chell, an 

 American Trenton species very much related to Raf. alternata Emmons, 

 chiefly differing by greater convexity and smaller size. On finding the 

 character of the ventral muscular area in a Norwegian specimen, it was 

 evident that the two forms could not be united, R. minnesotaensis as well 

 as R. alternata having very large muscular fields in the ventral valves, 

 their length being about - /3 of the total length of the valve. The form 

 comes extremely near to M'Coy's species Strophomena iiugula from the 

 British Caradoc, a form, however, that according to the descriptions and 

 illustrations given by M'CoY and Davidson has a very fine surface line- 

 ation and the presence of distinct concentric corrugations as typical 

 characters, in these respects showing a difference. 



Occurrence. 5b, Lodviken, Helgø, Mjøsen district. 



A form that certainly is extremely nearly related to the form described 

 above but of which the material is too poor to give any exact knowledge 

 of the different characters, is found on the shores of the Rands- 

 fjord, south of Reikenvik in Hadeland in layers of sandy lime- 

 stone in shale of stage 4, probably 4 c (much resembling the Cyclocrinus 

 shale of the Mjøsen district). A ventral valve is depicted in pi. I, fig. 13. 



The outline of the specimens found is relatively somewhat broader 

 than in the Mjøsen form, but in other respects they seem to be alike. 

 The muscular area, the borders of which have been made visible by the 

 aid of acid, is like that of Raf. mjøsensis. 



