1915- No. 12. THE STROPHOMEMDAE OF THE KRISTIANIA REGION. 27 



more expressed in smaller shells than in larger. In the first mentioned 

 the ratio of height : length of ventral valves is about i : 4 whilst in a large 

 ones it measured as low as 1:9. Dorsal valves much flatter, large ones 

 often practically flat, except for an elongated concavity corresponding to 

 the elevated umbonal part of the ventral valve. 



The radiating striation of the surface varies very much in different 

 specimens. We always meet with striae of different sizes, and usually 

 the principal ones are fairly equal, prominent, rounded, and relatively 

 broad. In the interval of these we may again find ribs of highly differing 

 sizes, all, however, much less prominent than those of the first order. 

 Those in the middle of the intervals are generally stronger .than those 

 nearer to the main ribs. Every second rib may also be the stronger. 

 The width of the intervals and number of minor ribs also varies greatly 

 in different specimens and also to some extent in different places of one 

 specimen. As extremes we may have from i to 8 — 10 minor ribs between 

 two main ribs. As a rule, those radiating striae are crossed by extremely 

 fine crowded concentric lines, only to be seen in the best preserved 

 specimens. A feature that usually can be seen in well-preserved speci- 

 mens, both in 3-ounger ones and full-grown, is an indication of a faint 

 concentric wrinkling along the hinge linge, but only there. The wrinkles 

 are directed ver\- obliquely to the hinge, their direction depending on the 

 outline of the shell at the lateral extremities, the wrinkles being pointed 

 very strongly outwards where the cardinal angles are very acute. This 

 wrinkling may be seen also in the dorsal valves, where usually the surface 

 markings are much less distinct, and the radiating ribs more faint than in 

 ventral valves. 



As regards interior characters, the ventral muscular area can be seen 

 in a number of interior moulds of ventral valves, where the shell substance 

 {that is fairly thick) is removed. The muscular field is very well limited 

 in the posterior half, forming there a triangular depression that be- 

 comes shallower anteriorly and has straight sides, but otherwise it passes 

 gradually into the rest of the shell. No distinct anterior border is visible 

 in any specimen. A radiating striation of the muscular field is indicated 

 in the best preserved specimens. 



Observations. This form certainly has to be identified with 

 that common British species, occurring in corresponding beds, exactly 

 agreeing in all main characters. Judging from the illustrations Davidson 

 has given in his monograph, (pi. XL\', figs, i — 10) there seems, however, 

 to be a minute point of difference between the British and Norwegian 



