26 OLAF HOLTEDAHL. M.-N. Kl. 



The surface markings consist of radiating ribs of two orders, the 

 major ones very prominent, sometimes seeming especially strong as the 

 surface of the shell has there a faint elevation. In spite of the number 

 of minor ribs in each interspace being rather low, generally 2 — 4 (in the 

 anterior half of the shell), the distance between each two of the major 

 ones is as much as i^/o — 2 mm. as the intervening ribs are distinctly 

 broader than the minor ribs usually are in Strophomenids. When the 

 surface is well-preserved a series of crowded minute concentric raised lines 

 can be seen. A few lines of growth are usually present but there is no 

 indication of a real corrugation. By removing the shell substance of a 

 ventral valve with acid, the outline of the muscular field became visible 

 (not very clearly) and proved to be very small, its length being about 

 5 mm., while the length of the valve was 18 mm. 



Observations. The form described above is a rather charac- 

 teristic species, general form and other characters (except size) being very 

 constant in all specimens, from the smallest to the largest. Very typical 

 is the keel-like elevation in the median line, a character expressed in the 

 name proposed. It differs distinctly from all other Norwegian species and 

 as far as I know also from foreign species previously known. 



Occurrence. 5a, Stavnæstangen, Ringerike. 



Raßnesquina, expanse Sowerby. 

 (PI. II, fig. 6-13.) 



Synononiy see Strophotnena expansa Sow. in Davidson, British Silurian Brachiopoda, 

 p. 312. 



Mat. pres. This is one of the few species in the material that is 

 represented by a really large number of specimens, these however never 

 been found free, but always embedded in rock. 



Description. There are present specimens of very different 

 stages of growth, the largest measured having a greatest width of about 

 43 mm., a length of 35 mm. The outline is very well and regularly 

 rounded, the lateral sides being for some distance parallel or even slightly 

 convergent posteriorly. Then, near the hinge-line the shell widens and 

 merges into cardinal extremities of different acuteness. Sometimes they 

 are rather obtuse and only slightly prominent, but I have also seen them very 

 sharp reaching far beyond the rest of the shell. The proportion of width 

 to length varies from 10 : 7 to 10 : 9. The convexity is gradual and 

 regular, yet varying strongly, greatest in the umbonal region which is 

 elevated above the rest of the shell. The convexity is generally much 



