1915- No. 12. THE STROPHOMEXIDAÊ OF THE KRISTIANIA REGION. 57 



or a little less. Ratio width — length considerably varying. The follow- 

 ing figures give the dimensions in mm. of three specimens: 



Width 12 24 30 

 Length 7^ 2 14 20 



Specimens preser\-ed in limestone matrix show a distinct concavity, 

 very different in different specimens. Ventral valves have a slight eleva- 

 tion near the beak, the rest of the valve being slightly concave. Dorsal 

 valves slighü\- or more distinctly convex as in the specimen depicted in 

 pi. X, fig. 4. The strongest curving is found near the antero — lateral 

 margins. In the circumbonal portion of the dorsal valve a slight elongate 

 median depression is generally seen, while near the front there some- 

 times appears to be preseni; a broad fold-like median elevation. In speci- 

 mens with the last mentioned character the outline is more triangular than 

 in the others. Several specimens from different districts with shale as 

 matrix are not at all curved, but — roughly speaking — flat. 



The surface ornamentation is very characteristic. We have a series 

 of thin but well raised radiating ribs at fairly equal intervals (in the por- 

 tions near the rounded border on an average i — i^ o ^^- apart) and in these 

 intervals a lattice-hke structure, with crowded extremelv delicate radiating 

 and concentric lines of about equal size crossing each other. In addition 

 we find that the surface of each interval between the major radiating ribs 

 shows alternating small elevations and depressions, those of two adjacent 

 interspaces generally not corresponding, and thus not giving rise to con- 

 tinuous corrugations. These depressions and elevations are less distinct 

 near the rounded border than in the circumbonal region. 



The shell substance of this species is remark!}^ thin and delicate, and 

 so are the internal ridges and processes of the articulation apparatus 

 and muscular area. It is therefore difJlcult to obtain any very distinct pic- 

 ture of the internal character b\' treatment ^vith acid but an indication of 

 some features has been found. In the ventral valve the muscular area 

 must have been very small. In a specimen with width at least 22 mm. the mus- 

 cular area was only 4 mm. wide. The ridges bordering the area are dis- 

 tinct near the hinge, where the\- diverge at about a right angle, soon how- 

 ever changing their direction so as to proceed straight forwards. No in- 

 dication of a ridge to the front of the muscular impression can be seen. 

 In a mould of the interior of a dorsal valve two ver\' small rounded 

 cavities, corresponding to the branches of the cardinal process, can be 

 seen, and from these holes a pair of short and delicate depressions, cor- 

 responding to the crural plates, diverge at about an angle of 120''. ra- 

 pidly disappearing laterall}-. 



