I9I5- ^^O- 12. THE STROPHOMEXIDAE OF THE KRISTIANIA REGION. 67 



When the cardinal extremities are preserved they are well projected and 

 sharp. The curvature of the valves also varies much. The deflection of 

 the marginal portion may be more acute or more rounded and the relative 

 size of the central disc (that is always slightly convex in ventral, concave 

 in dorsal umbonal region) and deflected portion excedingly different. In 

 one specimen, the length of the marginal rim (along the median line) is 

 the longer, but generally it is inferior to the median length of the flatter 

 portion. We may have as an extreme a specimen like that depicted 

 in pi. VI, fig. 5, where the deflected portion is only ^3 of the total 

 length of the valve, the deflection also being verN' gradual and- not 

 sharply marked. 



As to the surface ornamentation, the number of finer ribs in the 

 inter\als between the major ones varies very much in different specimens. 

 and often also in the same specimen. It is generally larger in the middle 

 part of the shell than close to the rounded margin, this being due to the 

 greatly increasing number of principal ribs towards the border. Of con- 

 centric corrugations not the slightest indication is seen. 



Occurrence. 7c, 8a. gd. Ringerike. Asker-Bærum, Bundetjord, 

 Holmestrand. 



Schuchertella Girtv 1904. 



Schuchertella pect en Linn. 



(PI. XI, fig. I -7.1 



For synonomy see Strophotttetui ptcten in Davidson, Brit. Sil. Brach, p. 304. 



Mat. pres. A large number of ventral and dorsal valves in rock, 

 very few o( which, however, are complete. 



Description. This fomi occurs through a considerable thickness 

 of strata, and the specimens found in different .stages show a somewhat 

 different character, however, not so much as to be separated as distinct 

 types. 



From Stage 5 three incomplete small specimens (length of hinge line 

 about 14 mm.) are found that undoubtedly must be considered as pre- 

 cursors of the larger Schuchertella pecten. Outline, convexity and the 

 typical coarse striation are exactly like that in the young specimens from 

 higher stages. 



The specimens from stages 6 and 7 are of rather small or medium 

 size, largest specimen measured with width at hinge 34 mm., length 23, 

 of semi-elliptical rather broad outline, posterior parts of lateral margins 

 about parallel with very small, slighth- projected cardinal extremities. 

 Radiating striation rather or very coarse but differing somewhat in detail. 



