1915- No. 12, THE STROPHOMENIDAE OT THE KRISTIANIA REGION. 7 1 



ordinary smaller specimens. Another distinguishing character is the well- 

 expressed convexity of the greater part of the central disc of the ventral valve of 

 the large specimens, a corresponding concavity being, or not being, met 

 with in the dorsal valve. A number of evenly convex Strophomenid 

 fragments with regular concentric corrugations, (see pi XII, fig. 2), at first 

 thought to represent a distinctly varying Leptaena, are probably only the 

 circumbonal portion of this convex type of L. rhoniboidalis. the deflected 

 margin being broken and hence not observed. 



Another type from 8 c, Bundetjord, is that depicted in pi. XII, fig. 

 II. We posess several specimens, very much alike. They are small, have a 

 rather convex central disc, the outline of which is comparatively very long 

 and narrow, about as long as it is wide. Compared with the length of 

 the central disc, the median length (width 1 of the deflected margin seems 

 to be rather short. The exact frontal border is however, not seen in any 

 specimen. The concentrric corrugations are rather faint. As to the radi- 

 ating striation a number of striae in the median portion are more con- 

 spicuous than the rest. 



Both ventral and dorsal interior characters are seen in specimens from 

 stage 8. The oval ventral muscular field (see pi. XII, fig. 5) is bordered by 

 a strong ridge nearly all around. The elongate adductor scars in the 

 median and anterior portion of the muscular field are very distinct. The 

 interior of the dorsal valve pi. XII. fig. 10. shows the well-known features 

 typical of this species. : 



Stage 9. Figs, i, 9, 12. The dominating type here is a medium 

 sized form, characterised by the central disc of the ventral valve being 

 rather convex and passing more gently — with less sharp geniculation — 

 into the deflected margin than in the ordinary, older types. The dorsal 

 valve has a more flat central disc and sharper geniculation. The concen- 

 tric corrugations are generally very faint, sometimes nearly invisible. 

 This type is found in its extreme development in 9 a in Ringerike, simi- 

 lar specimens being found there alreadv in the upper part of 8. 



If we sum up the results of the studies on the occurrence of L. 

 rhomboidalis we have the following facts. The greatest uniformity in 

 exterior appearance is found in the middle portion of the succession of 

 the region, in stages 5 — 7. the type here being characterised by subrec- 

 tangular central disc, flat or concave central disc of ventral valve and 

 prominent concentric corrugations. In stage 4, the specimens show a 

 great variety in general form, being however more triangularly shaped 

 than the younger ones. In 8 and 9 we meet several types, of dift'erent 



