1915- No. 12. THE STROPHOMENIDAE OF THE KRISTIANIA REGION. 1 7 



suffered from pressure. On the other hand we find relatively short and 

 broad specimens, with well-rounded outline in front. In an extremely 

 broad specimen the width measured 22, the length (as seen from above) 

 12 mm. In many specimens a slight lobation of the deflected margin, 

 especially close to the front, may be seen, giving an impression of a 

 faint fold in the ventral valve. 



The convexity also varies a good deal, the broader specimens usually 

 showing a more regular curving, whilst the longer specimens have the 

 marginal border more abruptly deflected. This deflected margin is gene- 

 rally a little shorter than the central disc that in the ventral valve is 

 always distinct!}- convex, especially in the umbonal region, in dorsal valves 

 nearly flat, or only very slightly concave. 



Of surface markings we have a series of distinct radiating ribs, with 

 from 3 to 6 finer ones between each pair. The stronger ribs are espe- 

 cially conspicuous in the median portion of the shell, and most marked 

 in front. Sometimes a single rib situated in the very median line is st 11 

 more distinct than the rest. In dorsal valves the difference in size of the 

 radiating striae seems to be less strongh- marked. In well-preserved por- 

 tions o*' the surface, crossing the radiating ribs, both primary and secon- 

 dary, we see a series of extreraeh' crowded raised lines, the distance 

 between which is considerably less than between the minor radiating 

 striae. Of real concentric wrinkles such as are found in typical specimens 

 of Raf, dcltoidca ver}* few are seen. In many species they are totally 

 absent, in others they are indicated near the hinge line. 



Interior characters unknown. 



Observations. In spite of the form described above showing 

 minor differences from American t\-pical specimens of Raf. deltoidea I do 

 not for the present think it necessary to keep it apart as a distinct variety. 

 The concentric corrugations may also be obsolete in the American speci- 

 mens thut are referred to the species. The fact that the bulk of our 

 specimens have a more sharply angular outline than the typical Raf. del- 

 toidea is neutralised b}- the presence in the material of specimens that have 

 an outline as rounded and even broader and shorter than in typical 

 American specimens. 



Occurrence. 4b (transition 4b a — 4bp?| Bratterud, Ringerike. 



Asker— Bærum (figs. 2— 3 1. 

 Mat. pres. A few ventral valves and outer moulds of dorsal 



ones in rock. 



Description. The specimens from Asker — Bærum differ in 

 several respects from those described from Ringerike. The}' are -mailer 

 Vid-Selsk. Skrifter. I. M.-N. Kl. 1915. .\o. 12 J 



