66 OLAF HOLÏEDAHL. M.-N. Ki. 



The surface marking consists of radiating ribs of fairly uniform size. 

 In the umbonal part of the valves they are generally all alike; a difteren- 

 tiation then begins towards the antero-lateral margins. For some distance 

 from the beak every second rib is generally stronger than the rest, hi 

 the zone nearest the front we generally have a larger number, mostly 

 2 — 3 between two of the stronger ones. The difference in size is, how- 

 ever, by far not so striking as in Strophonclla eiiglypha. Concentric corru- 

 gations are not present, and only a few indistinct lines of growth are 

 seen. The ordinary very fine, crowded, concentric, raised lines have not 

 been observed. 



The interior of the ventral valve is naturally weathered out in a large 

 number of specimens, some of which are shown in pi. VI, fig. 8. The 

 muscular field is heart shaped, distinctly bordered all around, the surround- 

 ing ridges being especially prominent in the posterior part. A distinct 

 median septum is present, on both sides of which in the posterior ^/^ 

 of the muscular field the very narrow, strongly elongate adductor scar 

 is seen. 



The ventral cardinal area that can be well studied, is distinctly trans- 

 versely striated for a distance of 3 — 5 mm. on each side of the deltidium. 

 The striae that cross the area somewhat obliquely are especially strong 

 in the broad (high) portion of the area nearest the middle, gradually dis- 

 appearing laterally. Interior of dorsal valve is not known. 



Observations. This species comes in several respects near to 

 Strophonella euglypha, but can be distinguished by the following characters: 

 outline much rounder, not triangular, less curved valves and more evenly 

 curved without abrupt deflection of the antero-lateral part, radiating striae 

 of more equal size, the finer ones being not so delicate as in S. euglypha. 



An American species that remind sone a good deal of A. euglyphoides, 

 is Amphistrophia patenta Hall from the Clinton of New York, Ohio and 

 other states. 



Occurrence. 9 (a — c), Ringerike; Asker-Bærum; Holmestrand. 



Strophonella euglypha, His. 

 (PI. VI, fig. 5-6.) 



For synonomy see Strophoniena euglypha Hisinger in Davidson, Brit. Sil. Brach, 

 p. 288. 



Of this well known form, so common in the Siluric rocks of northern 

 Europe, a fairly large material is present, also as free specimens. The 

 size of the specimens varies considerably, and so does the outline, .some 

 being distinctly sub-triangular, others more broadly rounded in front. 



