42 OLAF HOLTEDAHL. M.-N. Kl. 



A characteristic feature of the shell is that the surface near the frontal 

 margin has an undulating character; in the median line we find in the 

 ventral valves a flat fold, on each side of that a flat sinus, and further to 

 the sides — particularly distinctly developed in one of the specimens — 

 a new, broad, and flat fold. 



Surface markings (seen in one specimen) consist of very delicate 

 radiating ribs, every other one of which seems to be a little stronger than 

 the rest. The difference may sometimes be so minimal that the ribs may 

 be said to be equal. In one specimen a few very faint concentric wrinkles 

 can be seen near the hinge in good light. 



The interior mould of the two ventral valves shows a triangular mus- 

 cular area, very strongly marked in the posterior part, forming here in 

 the cast a somewhat highly elevated triangle, with an angle at the hinge 

 of about 80*^. Anteriorly the muscular area passes without any particular 

 marking into the remainder of the surface. 



In one of the specimens we are able to see a faint depression in the 

 median line, reaching from the angle of the muscular field and forwards 

 5 — 6 mm. and qt a distance from this median line, of ca. 1,5 mm. another 

 parallel line on each side. Still further outwards indications of other 

 depressed lines may be seen. The central lines correspond with the 

 median septum of the muscular field, the lateral marks with the longi- 

 tudinal ridges typical in diductor scars of the Leptostrophia-ïorms. (Com- 

 pare e. g. the illustration of the interior of a ventral valve of Z-. Becki Hall 

 (Low. Helderberg of New York) in Hall and Clarke's »Introduction«, 

 pi. XIII, fig. 23). 



Observations. Also this form comes very near to Strophomena 

 filosa Sow. of the Wenlock and Ludlow division of the British Isles. The 

 extreme flatness, and the fine radiating ribs are very characteristic of boht. 

 Furthermore there certainly is a considerable correspondance in the 

 character of the ventral muscular area. There are, however, also distinct 

 distinguishing features present, especially the far greater width in relation 

 to length in the Norwegian form, the proportions in the latter being about 

 2:1, in the British 4:3. Further, the marginal undulations that are 

 characteristic of the Norwegian specimen are not present in the British, 

 as far as can be judged from existing illustrations and descriptions. 



Occurrence, gd, Gaaserumpen and Lange off" Holmestrand. 



