1915- No. 12. THE STROPHOMEMDAE OF THE KRISTIANIA REGION. 



77 



being much coarser than those found in typical Plectambottitcs forms, from 

 younger beds. 



In size and outline this forms reminds one somewhat of jEos/ro^/iowe/wa 

 elegantiila Walcott [Strophontena IValcotti Moberg and Segerbergi that 

 is found in the Ceratopyge division of Southern Sweden, but it has a 

 rather different surface ornamentation. 



Occurrence. 3c/!?, City of Kristiania. 



Plectambonites c\. papulosa Cowper Reed. 

 (PI. XVI. fig. 3.1 



Mat. pres. A fairly large number of specimens in rock. 



Description. Shell small (even for a Plectambonites) outline 

 semicircular, nicely rounded. Cardinal angles not sharp, only slightlv less 

 than 90°. Width in adult specimens twice the length (dimensions in a 

 rather large specimens 6 and 3 mm.i. younger specimens relativelv longer, 

 one that was measured had a width of 3, a length of 2 mm. I. Convexitv 

 not strong, most prominent near the beak, near the rounded margin nearly 

 flat. The radiating striae are rather sharp and prominent for the size of 

 the shell. They are of fairh' uniform size, and situated without distinct 

 intervals. The number of ribs at the rounded margin is in adult specimens 

 about 70, in small ones much less. The number of ribs increases towards 

 the front in part by bifurcation, in part by intercalation. 



Observations. There is no doubt that this form from the 

 Trinucleus division of several districts of the Kristiania region is very 

 much like the Plect. papulosa described by Cowper Reed from the Slade 

 Beds of the Trinucleus seticomis division of the Havertbrdwest district '. 

 They are both characterised by small size, and by rather equal radiating 

 ribs, in this feature differing greatly from the more common Plectambonites 

 forms of the middle and higher Ordovicic. As, however, I have not been 

 able to decide if another of the typical characters of the British species, 

 the very coarsely papillose interior, is present in the Norwegian specimens, 

 1 will not make any final decision as to the identity of the two. A small 

 difference seems to be present also in the general outline, the cardinal 

 angles of the British form being more sharp. 



Occurrence. 4c (Upper part of Trinucleusshalel. Railroad section 

 Roa — Lunner, Hadeland: Frogno. Ringerike. 



1 Geol. Magazine Dec. V. Vol. II. 1905. p. 457, pi. XXIII, tics. 13. 14, 15. 



