1915- ^*^'- !-■ THE STROPHOMENIDAE OF THE KRISTIANIA REGION. 81 



Occurrence. Coelosphæridiumzone, Mjesen district (?); 4 c (upper 

 parti, Lunner, Hadeland. 



Plectambonites sericea Sow. 

 (PI. XIV. fig. 1—3.) 



For European Synonomy see Lfptœna sericea Sow. in Davidson, Brit. Sil. Foss., 



P- 323. 

 For .\merican Sv'nonomy see Plfctaniboniteus srriceus Sow. in Schuchert, Sj'nops. 

 Am. Foss. Brach., p. 310. 



This well known tbrm is exeedingly common in the upper Ordovicic 

 of the Kristiania region. Specimens from difterent horizons and 

 districts show relatively very constant features in all respects. In all 

 specimens where the exact outline could be seen the cardinal angle was 

 distinctly angular (in some of Davidson's figures they are rather rounded), 

 the angle being somewhat different, var3'ing from 90° to about 75°. The 

 number of finer striae in the intervals between the major ones is generally 

 2 — 4. The interior characters can be seen in a great number of moulds, 

 where the matrix is sandy. The anterior borders of the dorsal muscular 

 impressions are never very distinctly bordered, but can nevertheless be 

 seen where the state of preservation is good. The length of the impressions 

 is about ^ :, of the total length of the shell. 



Occurrence. The typical form is known from both Cyclocrinus- 

 horizons in the Mjosen region, from corresponding zones in the Northwestern 

 part of the Cambro-Siluric district of Hadeland, and further from 5 a, 

 Mjosen district and Ringerike, 5 b Ringerike and Skien — Langesund district. 

 This form is especially commonly found in the Northwestern parts of the 

 Kristiania region, the Mjosen district. Hadeland and Ringerike. Besides 

 the material from the higher part of 4 and from 5 similar specimens 

 generally of smaller size are found in 4 b, Bundetjord and Ringerike, the 

 material being not sufficient for a definite decision as to their systematic 

 position. 



Plectambonites Schinidti Tql. var. precedens n. var. 

 (PI. X\'. fig. 10 — II. I 



In several districts are found from 4 b (and 4 c ?l specimens of a 

 Flectambouitcs that strong!}- remind us of the large typical F. Sclunidti 

 from stage 5, mentioned below, being however much smaller. As is the 

 case with P. Schmidtt, this older form also varies very much in outline 

 and convexity. We have relatively flat types (always more convex than 

 P. sencea) and we have globose types. As a typical representative of the 

 Vid.-Selsk. Skrifter. I. M.-.\. Kl. 1915. Xo. 12. 6 



