48 OLAF HOLTEDAHL. M.-N. Kl. 



typical feature is the not strong, but invariably present concentric, irre- 

 gular corrugations in the central disc, especially conspicuous near the 

 hinge line. 



In the interior of the dorsal valve the crural plates, diverging at about 

 120'', are very broad and solid, running straight for a length of 3 — 4 mm. 

 then gradually disappearing. 



Observations. Strophomena pseudodeltoidea comes in exterior 

 characters exceedingly near to the American Strophomena fluctuosa Bill., 

 that I have been able to study in specimens (from Spring Valley, Minne- 

 sota ') given me by Prof. Schuchert. The only difference I can see is 

 that the cardinal angles in the American form are less projected and sharp 

 and the major ribs placed at smaller intervals. From the figure of the 

 interior of the dorsal valves in Wln'chell and Schuchert's paper in 

 Paleontology of Minnesota, part I (pi. XXXI, fig. 17) it also seems as if 

 the crural plates are comparatively thinner in that American specimen. 



Of other European species, Strophomena pseudodeltoidea in general 

 form and surface markings strongly resembles the 5. (?) Jentzschi Gagel, 

 described above, the latter being, however, a smaller species. Of other 

 distinguishing character can be mentioned the somewhat broader outline (of 

 the central disc) and the more uniformly sized radiating ribs of that form. 



Occurrence. 5a, Vestre Svartø and Stavnæstangen, Ringerike. 



Strophomena, sp. (cf. S. pseudodeltoidea, Stolley). 

 (PI. VI, fig. 4.) 



A single but fairly complete specimen showing very much likeness to 

 the form just described, but also so much difference that it must be kept 

 separate. 



The circumbonal fairly flat portion is relatively much broader and 

 the curvature of the valves, in longitudinal section, much stronger than in 

 all specimens seen of Strophomena pseudodeltoidea. 



Surface characters can be indistinctly seen in the posterior part of 

 the dorsal valve. At about the place of the strongest convexity we ob- 

 serve a series of fairly broad, radiating ribs, with 7 — 10 finer ones between, 

 and crossing these fine and crowded, concentric, raised lines. In the 

 central disc are seen in a good light faint concentric corrugations, that 

 certainly would have been more distinct had not the surface been worn. 



Occurrence. 6a, Hvalsbakken, Asker. 



^ This type has recently been separated by Foerste as var. occidentalis. 



