68 OLAF HOLTEDAHL. M.-N. Kl. 



Generally all the striae are of about equal size, the number increasing 

 towards the antero-lateral margins by interpolation. In some specimens 

 we may find every second rib of somewhat minor size, and also speci- 

 mens with two finer ribs between two major ones are seen, the distribution, 

 however, never being very regular. 



The bulk of specimens have their origin in zone 8 c (in Malme). 

 As will be seen from the illustrations, these specimens are of very large 

 size in adult stages, with a very varying outline. We find regularly 

 semi-circular extremily broad specimens and we have others with the width 

 scarcely exceeding the length. Dimensions of a number of specimens are 

 given below in mm.: 



Width 63 (.■'I 62 5o(?) 39 

 Length 32 34 36 36 



Cardinal angles generally slightly less than go''. Ventral valves ele- 

 vated near the beak, the rest of the surface very slightly concave, some- 

 times only concave near the rounded border; dorsal valve evenly and 

 moderately convex except near the cardinal angles, that are nearly flat. 



Radiating striae coarse, not very crowded, mostly of fairly uniform 

 size, sometimes one or even two of distintly inferior size coming between 

 two stronger ones. Crossing the striae as well as intervals are seen 

 fairly distinct, undulating raised lines, of which about 4 or still more occur 

 in one mm. No corrugations are observed. 



The interior of the ventral valve shows a muscular field, the posterior 

 triangular portion of which is well bordered by distinct ridges, the anterior 

 margin being indistinctly marked in the few specimens, showing the interior. 

 Dorsal valve with the branches of the bilobed cardinal process rounded 

 triangular in section, crural plates very strong, short, ending abruptly. 

 On each side of the median line in front of the crural plates are seen 

 indications of longitudinal ridges. 



Observations. In spite of the difference in size in the speci- 

 mens from 8 and those from 6 (and from 5) I cannot see they can be 

 distinguished. I have been able to study the interior character of dorsal 

 valves of small specimens from 6, but they were entirely similar to the 

 large one from 8. A Swedish specimen from Gotland that I have seen 

 is in size like the larger ones from 6, while Dalman's old figures show 

 specimens, somewhat larger, like moderate or small sized specimens from 

 8 c. Gagel's species Strophomejia Lindströmi (see Brach, d. cambr. u. 

 sil. Geschiebe etc.) p. 43, pi. 'II, fig. 12, as far as can be judged from his 

 description and illustrations, seems to be very much like the broader 

 types from 8, except for the radiating ribs being more crowded. 



