62 OLAF HOLTEDAHL. M.-N. Kl. 



It certainly also shows resemblance to the somewhat older form just men- 

 tioned above. 



Ventral valve is nearly flat, except for a distinct sinus at front, dorsal 

 slightly convex, with broad keel-like longitudinal elevation in anterior 

 half. In places where the surface is not too strongly worn a system of 

 not very straight medium sized radiating ribs of fairly uniform size are 

 seen. A short distance from the frontal border there generally occur 

 3 — 4 in a transverse space of 2 mm. What most strongly characterizes 

 the surface of the shell are the very prominent concentric rounded ridges 

 and depressions, marking lines of growth. This character is here much 

 more developed than in the Strophomena coslatula, that besides seems 

 to differ by less broad outline and coarser radiating striae. 



Occurrence. 4 c, South of Reikenvik, Hadeland. 



Strophomena, (?) coslatula, Hall and Clarke. 

 (PI. XIII, fig. 6—7.) 



1894. Strophonella costatida Hall and Clarke. Palæontology of New York Vol. VIII, part 

 II. P- 359. P'- LXXXIV, figs. 15-16. 



Mat. pres. Two fairly complete specimens, besides a few of a frag- 

 mentary nature. 



Description. Shell of fairly small size, outline semielliptical, ratio 

 width: length varying as shown by following figures : 



Width 24 mm. 22 mm. 

 Length 25 — 17 — 



Cardinal extremities not quite complete in any specimen. They cer- 

 tainly have been only slightly conspicuous, the lateral margins being sub- 

 parallel nearly immediately behind the hinge. Ventral valve slightly ele- 

 vated in the umbonal region, the rest of the surface very slightly concave. 

 Near the front a sinus of different size in the different specimens. Dorsal 

 valve nearly flat for a small portion near the hinge, otherwise moderately 

 convex with an anterior fold more or less distinct. 



Dorsal cardinal area rather small, lying nearly horizontal, ventral car- 

 dinal area broader, at an angle of about 120'' to the horizontal plane of 

 the shell. Deltidium fairly convex, apex perforated by a minute circular 

 foramen. 



Surface with coarse, irregular radiating ribs without flat intervals 

 between, their number increasing towards the antero-lateral margin chiefly 

 by bifurcation. A series of coarse concentric lines of growth is con- 

 stantly present. 



