J 915- ^O- 12. THE STROPHOMENIDAE OF THE KRISTIANIA REGION. 79 



Plectambonites cf. quinquecostata M'Coy. 

 (PI. XV. fig. 7—8.) 



A small number of general!}' very fragmentary specimens from stage 5 a 

 have the same t\-pical surface features with only a few relatively strong 

 radiating ribs in the posterior portion, the number increasing by inter- 

 calation towards the rounded front, but differing considerably from the 

 specimens described above and from the British specimens of Pi quinquc- 

 costata as depicted by Davidson and others, by a much more expressed 

 convexity, both in longitudinal and transverse section, the median portion 

 of the ventral valve being very much elevated, with steeply sloping sides. 

 The convexity may. however, be rather different in different specimens, 

 rhe outline is also different, relatively longer, more triangular, and the 

 size generally smaller. One very narrow specimen measured 6 mm. in 

 both width and length ; generally the width is somewhat more, the length 

 somewhat less. 



Occurrence. 5a, \'estre Svarto and Stavnæstangen. Ringerike. 



Plectambonites trabeata Lindstr. var. acuminata n. var. 

 (PI. XIV, fig. 6—7.) 



Mat. pres. A tew outer and inner moulds of ventral and dorsal 

 valves present. 



Description. In size and outline much like PL scricea, width 

 twice the length or somewhat less. Cardinal angles rather sharp. Convexity 

 of median strength, rather great near the beak. Radiating striae of two 

 distinct sizes, the distance between the major ribs in the specimen with 

 best preserved surface characters generali}- 1.5 — 2 mm. near the front, 

 thus larger than in Pi. scncea, and the number of finer striae in each 

 interval about 10. A very characteristic surface feature is the delicate, but 

 always distinctly visible, concentric wrinkles, that cross the valve as 

 regularly arranged, fairly continuous undulations. 



The interior characters are much like those in PL scricea, the ventral 

 muscular field however being somewhat smaller. The posterior, main 

 branches of the vascular system are very distinctly seen, crossing the 

 muscular areas longitudinally. The muscular impressions of the dorsal 

 valve are oval in shape, smaller than in PL scricea, their length being 

 less than half the length of the valve. They have been crossed longi- 

 tudinally by a fairly strong median ridge besides less conspicuous ones. 



