72 OLAF HOLTEDAHL. M.-N. Kl. 



sizes, characterised by rather convex central disc; the principal type of 9 

 having the corrugations very faintly developed. The types do not 

 seem to possess so distinguished and fixed characters that a naming of 

 them is justifiable. 



As to the interior characters there seems to be a difference in the 

 ventral valves between the older and younger specimens, the total ventral 

 muscular field as well as the adductor scars being much more strongly 

 marked in the latter. 



Occurrence. L. rhomboidalis is found in all the districts of the 

 region, the total vertical range being 4 b — 9 e. 



Leptaena Kjeralß nov. sp. 

 (PI. XIII, fig. 9— II.) 



1865. Strophomena sp. n. Kjerulf, Geologisk veiviser ved excursioner i Christiania omegn, 

 p. 12. 



Mat. pres. A rather small number of incomplete ventral and 

 dorsal valves in rock, besides fragments of free specimens. 



Description. Shell very large, outline semicircular, broadly 

 rounded in front, cardinal angles (not distinctly seen in any specimen) 

 probably only slightly projected. The shell is geniculated as in Leptaena 

 rhomboidalis, but not so sharply as in the typical specimens of that species; 

 the central disc, that is nearly flat, only slightly convex in the umbonal 

 region (concave in dorsal valve), passes more or less gently into the 

 deflected margin. The length (width) of the deflected margin in the median 

 line is in one specimen about -/3 of the length of the central disc, in 

 another only about ^/o, as far as can be judged from the not very well 

 preserved specimens. 



The cardinal areas are seen in one specimen. The dorsal is very 

 small, the ventral that is longitudinally striated large, 4 mm. high at the 

 lateral extremities of the convex deltidium. The beak is perforated by a 

 very small circular hole. 



The radiating striation consists of a large number median-sized ribs, 

 which in the lateral portions of the valves are of fairly uniform strength, 

 while in the median part they are of two distinct sizes, 2 — 4 finer striae 

 coming between two stronger ones. The distance between the major ribs 

 near the front on an average is a little less than i mm. The most con- 

 spicuous feature is the very prominent and mostly very regular concentric 

 corrugations, that are extremely coarse, both deep and broad. The total 

 number of these corrugations in the large central disc is generally 8 — ^12. 

 The distance from the ridge of one wrinkle to another is sometimes as 



