56 OLAF HOLTEDAHL. M.-N. Kl. 



every second or third is stronger than the rest. What is most con- 

 spicuous in the surface markings of the ventral valve are the very pro- 

 minant, never absent lines of growth, often present in the form of strong 

 depressions and ridges. Concentric lines of growth are also present in 

 the dorsal valves, but are there by no means so coarse. 



Observations. Though the material of this form is not large, 

 it is nevertheless sufficient to give a rather exact idea of the general 

 exterior appearance. In the latter it seems to be rather different from 

 any described European form known to me, coming, however, exceedingly 

 near a North American species, viz. Strophomeyia nutans, of the Rich- 

 mondian of Ohio and Indiana. I have studied the exterior of this forms 

 through existing illustrations ^ and also in a small number of complete, 

 and well-preserved specimens (from Oxford, Ohio) given to me by Prof, 

 ScHucHERT. In size, outline, and radiating striation I do not think the 

 American and Norwegian specimens can be separated, the only difference 

 observed being in the circumstance that the former has the strongest 

 curvature in front of the centre of the valves, and a more marked deflec- 

 tion, while the Norwegian specimens have the greatest convexity (con- 

 cavity) nearer the beak. The corrugation-like lines of growth so strongly 

 developed in the ventral valves of my material, seem to be altogether 

 absent in the American ones, thereby giving the specimens a different 

 general appearance. 



This species is named in honor of the eminent geologist B. M. Keilhau, 

 professor of the University of Kristiania 1834 — 1857, who published the 

 first geologic map of the Kristiania region, an excellent one for that time. 



Occurrence. 5 b, Helge, Mjesen district. 



Strophomena rigid a Barrande. 

 (PI. X, fig. 1-4.) 



1879. Strophomena rigidn Barrande. Syst. Sil. du Centre de la Bohême, Vol. V. pi. 



48, VII. 



1880. Strophomena arachiioidea (Tørnquist) Angelin-Lindstrøm, Fragmenta silurica, p. 29, 



pi. XIV, figs. 41 — 42. 



M a t. p r e S. A fairly large number of ventral and dorsal valves in 

 rock. 



Description. Shell of moderate size, outline generally semiellip- 

 tical to semicircular, regularly and well rounded with cardinal angles 90*^ 



1 Amongst others, in the fairly recent paper by A. F. Foerste: »On Strophomena and 

 other fossils etc.« in Bull. Sc. Lab. Denison Univers. 1912. 



