I915- NO- 12. THE STROPHOMEXroAE OF THE KRISTIAXLA REGION. I9 



corrugations are not present except as mere indications in the central 

 disc, especially in the umbonal region, where they can be seen in a good 

 h'ght as cur\-ed shades. 



As to the interior characters of this species, the size and form of the 

 ventral muscular area is indicated in two of the specimens where the 

 shell substance has been partially removed. The muscular field is rela- 

 tively small, as will be seen from the figure. 



Observations. Exteriorly, this form much resembles Raf. imbrex 

 P.\ND. but is larger and differs also by smaller muscular scars fi-om 

 this older species. It may also show considerable likeness to Rjif. deltoidea 

 CoNR.\D, especially in the highly convex form that was described by 

 Conrad and Hall as Stroph. camerata, but differs distinctly in several 

 respects. 



TTie Strophomena imbrex (?) that is described and depicted by Billings 

 from Anticosti - I Hudson River group) may possibly approach this Nor- 

 wegian species. 



Occurrence. 5a, Frognø, \'estre Svarto and Vaker, Ringerike. 

 5 a. Skien — Langesund district. 



Raßnesquina mjesensis n. sp. 

 IPI. U. fig. 1—3.) 



Mat. p r c s . A fairly large number of ventral and dorsal valves 

 in rock. 



Description. Shell of rather small size for a Rafitiesqitina. 

 Outline semi-elliptical or rounded deltoid. Greatest width at the hinge, but 

 cardinal angles generally not much less than 90°. Width of a mediimi 

 sized specium 20 mm., length (total) 17 mm., width of another more del- 

 toid 22, length 20 mm. Ventral valves rather convex, greatest cur\-ature 

 in typical specimens nearer the fi-onto-lateral margin than the hinge line. 

 In a verj' convex specimen of decidedly subtriangular outline, the greatest 

 convexit}' is in the central part of the valve. WTiilst in ventral valves the 

 portion of the shell lying between the hinge line and the line of greatest 

 curvature is alwaj-s decidedly convex, the corresponding part of the dorsal 

 valve is nearly flat, especially near the hinge. A section of a dorsal 

 valve therefore will show a more markedly geniculation than in a ventral. 



The surface markings consist of rather coarse radiating ribs, of 

 somewhat different sizes but not ver\' regularly arranged. Ever}' second, 

 third, fourth, or fifth, mav be stronger than the rest. In the umbonal 



' Palaeozoic Fossils 1865, p. 128. 



