1915* No. 12. THE STROPHOMENIDAÊ OF THE KRISTIANIA REGION. 73 



much as 6 mm. In the most complete specimens the corrugations are very 

 regular and continuous from hinge and back to the hinge, in some frag- 

 ments, e, g. the one depicted in pi. XIII, fig. 1 1 they are more irregular. 



A portion of the interior of a dorsal valve is shown in pi. XIII, fig. 9. 

 From what can be seen in this specimen it is evident that the muscular 

 impressions strongly resemble those of L. rhoinboidalis, the form of the 

 prominent markedly cur\-ed ridges surrounding the adductors seeming 

 however to be somewhat different, making the adductor scars less circular 

 than in the said species. 



Observations. The form here described is certainly very nearly 

 related to L. rhomboidalis, but by a series of difierent, well fixed and 

 constant characters: decidedly large size, much coarser corrugation, differing 

 radiating striation, can be easily distinguished. 



Occurrence. 4b.:?, from a number of loc. in Asker, transitional 

 beds 4b« — 4b,i, Bratterud, Ringerike. A single fragmentary but typical 

 specimen is labelled Blilid, Toten o'. Mjesen district. If this is correct, the 

 horizon must be the Coelosphæridium zone. 



Leptaena cf. unicostata Meek and Worthex. 

 IPI. XIII, fig. I.) 



Mat. pres. A ventral valve and a fragmentary outer mould of a 

 dorsal sitting in the rock. 



Description. Small, semicircular, comparatively very broad and 

 short, the width being nearly- the double of the length. Cardinal angles 

 onl}- slightly acute. The posterior part of the shell is slightly convex, 

 geniculating rather gently, giving a vertical marginal part, the height 

 »length! of which amounts to about two thirds of that of the central disc. 



On the surface we find crowded radiating ribs, that in the lateral 

 portion are of about equal size, while in the middle we find at irregular 

 inter\als several that are stronger than the rest, the strongest one being 

 situated exacdy in the median line. .\ series of regular, but rather faint 

 concentric corrugations are seen in the central disc. 



Observations. This interesting little specimen certainly comes 

 near to the Leptaena unicostata Meek and Worthen ^ from the upper part 

 of the Hudson River Group of North America. Besides knowing that 

 species from description and illustrations I have had the opportunity oi 

 studying tj'pical specimens from Minnesota, given me by Prof. Schuchert. 



' Strophomena itmœsiata Meek and Worthen. Geol. Surv. Illinois, III. p. 335. pi. IV, 

 fig. II. 



