1915- No. 12. THE STROPHOMENIDAË OF THE KRISTIANIA REGION. 65 



more sharply concavely deflected at an angle of 120 — 140^. Dorsal valve 

 flat or slightly concave in the central dise, anteriorly following the curving 

 of the ventral valve. Entire shell very thin. 



Radiating striae of two sizes, 3 — 4 of the minor usually coming 

 between two stronger ones. As usual, the distance between the major 

 ribs is greater in relatively broad than in more elongate specimens. In 

 the latter it does not generally exceed 1 mm. In well-preserved portions 

 of the surface the ordinary very fine crowded, concentric, lines are seen. 

 A typical surface-feature in this form is the concentric corrugations that, 

 though very differently developed in various specimens, are never quite 

 lacking. In some specimens the corrugations are very distinctly seen all 

 over the central disc, being obsolete in the deflected margin; in others 

 they are visible near the hinge line only, where they curve strongly 

 outwards. 



In two dorsal valves embedded in rock the mould of the cardinal 

 area, when the shell substance was removed, showed on the inner margin 

 a denticulation in the central part of the hinge, reaching only a few mm. 

 on both sides of the middle. 



In the same specimens, the other characters of the interior have been 

 worked out. The cavities corresponding to the bilobed cardinal process, 

 the crural plates and the median ridge can be well seen (see pi. VU, fig. 6). 



Observations. This form has by Davidson been united with 

 Stropliomoia (Strophorxella) cnglypha from which it, however, distinctly 

 differs. The Strophonella f Amphistrophia) Davidsoni can be distinguished 

 from the typical 5". euglypha by a more triangular outline, less sharply 

 deflected margin, and finally by the presence of corrugations, altogether 

 absent in 5. euglypha. 



Occurrence. 6c, Malmø and Malmøkalven, Bundefjord. 



Amphistrophia, euglyphoides nov. sp. 

 (PI. VI, fig. 7-9.) 



Mat. pres. A fairly large number of mainly ventral valves in rock. 



Description. Shell of medium size, oudine semi-elliptical, broadly 

 and evenly rounded at front, lateral margins near the hinge parallel or 

 slightly converging posteriorly, cardinal angles being about 90° or a little 

 more. \'entrai valve slightly and evenly concave with a faint elevation 

 near the beak, dorsal valve slightly and regularly convex, flattened out 

 towards the cardinal angles. In some specimens an indication of a very 

 shallow broad sinus in the ventral, a corresponding fold in dorsal valve 

 in the frontal portion, has been observed. 



Vid.-Selsk. Skrifter. I. M.-N. Kl. 1915. No. 12. 5 



