1915- No. 12. THE STROPHOMENIDAE OF THE KRISTIANIA REGION. 53 



Stage 5. (PI. VIII, fig. 7-8.) 



A considerable number of both ventral and dorsal valves from different 

 districts are present. The outline varies a good deal, some specimens 

 being rather broad, like those described above from Ringerike and Hade- 

 land, others more narrow, though never so much so as in those mentioned 

 from the Coelosphæridium zone of the Mjosen district. The concavity 

 also varies considerabl}-, from being a little more concave than the older 

 types to flat or even slightly biconvex, the ventral valve in this case 

 having no concave curvature, but only a slight convexity in the umbonal 

 region. 



As to the surface markings, we may find specimens where the major 

 ribs have such large intervals, as e. g. in the specimens from 4 b, Ringe- 

 rike, but generali}' the distance is smaller, and the number of intervening 

 ribs smaller. Usually one of these, in the middle of the interval, is larger 

 than the rest but not attaining to the size of the primary ribs. There is 

 also a more irregular distribution of ribs of different size. 



The interior characters seem to be quite like those in older specimens. 

 Ventral muscular field distincth' bordered by ridges posteriori}- only, and 

 the dorsal valve with cardinal process and crural plates exactly like in the 

 specimens from 4 b. 



A character that seems to be somewhat different in the specimens 

 from 5 and the older ones, is the size, the younger specimens being 

 constantly smaller. The majority of specimens from 5 have a width of 

 less than 25 mm., the largest about 32 mm., older specimens being not 

 seldom more than 40. 



A single ventral valve in rock, depicted pi. IX, fig. 3 with size, 

 outline and concavity like those in other specimens from stage 5 differs a 

 good deal in surface characters, the principal radiating ribs being only 

 slightly stronger than the rest, and sometimes they are not at all prominent. 

 The other ribs are of many different sizes, always arranged with one 

 strong one between two less strong, and vice versa. The specimen in 

 this manner bears a considerable resemblance to a form like Stropliomoia 

 grandis Sow. from the British Caradoc, that has a much coarser surface 

 ornamentation than the ordinary types we have here referred to Stroplio- 

 mena Asniusi. 



Occurrence. 5 (a— b), Mjesen district; Ringerike; Asker, Bærum. 



Also from stage 6 a few specimens that must be referred to the 

 6". ^s;«M5/-group are found (see pi. IX, fig. 2). They are rather small 



