OLAF HOLTEDAHL. M.-N. Kl. 



(a typical specimen having a width of i6 mm., a length — as seen from 

 above — of 12 — 13 mm.), relatively shorter, more evenly rounded in front 

 and with distinct concentric corrugations in the central disc, sometimes 

 also indicated in the upper portion of the deflected margin. Near the 

 median plan of the shell we also in these specimens find radiating ribs of 

 two distinct sizes, whilst in the lateral parts the striæ seem to be of a 

 fairly uniform size. The fine concentric lines are much less conspicuous 

 than in the specimens from Ringerike. In some specimens they cannot 

 be seen at all in spite of the surface of the shell being well preserved. 



Observations. The form from Asker — Bærum has all the general 

 characters of the exterior of small specimens of the typical American Raf. 

 deltoidea. 



Occurrence. 4b d, Sandviken, Bærum. 

 4 b (Î, Ostøen, Bærum. 



Rafinesqaina, Panderi n. sp. 

 (PI. I. fig. I.) 



Mat. pres. Four fairly complete valves (at least one also with 

 dorsal valve in situ) in rock besides smaller fragments. 



Description. Shell of medium size, outer form much like full- 

 grown specimens of Raf. imbrex Pand. Shell roundly geniculated with 

 a slightly convex central disc, the length of which amounts two V2 or "/5 

 of the total length. Seen from above the shell (central disc) has a well 

 rounded semi-circular or subrectangular outline with cardinal angles, how- 

 ever, slightly projecting. Width of a medium-sized specimen 27 mm., 

 length (seen from above) 16 mm. Greatest width of the shell seems to 

 be at some distance in front of the hinge line, this greatest width, however, 

 only very .slightly exceeding the width at the hinge. 



The surface is not very well preserved- in any of the specimens, but 

 in the major features its character can be discerned. Of radiating ribs 



we have a series of stronger ones (the 

 distance between two of which in a 

 portion of the surface near the front, 

 where the preservation in one specimen 

 is good, being 1^/2 — 2 mm.) and in the 



Figure showing the size of the ventral 



muscular field of Rafimsquina Pander, interspaces between these 4—7 finer 

 compared with the size of the central ones. Fine concentric lines crossing 



disc and the length of the shell in the , , j i i i • i i 



.... „ . . ,^ , the other probably have existed; but 



median line. bpecimen iroin Vaker, ' -^ 



Ringerike. cannot distinctly be seen. Concentric 



