78 OLAF HOLTEDAHL. M.-N. Kl. 



Plectamhonites convexa. Pander. 

 (PI. XIV, tig. 9.) 



1830. Pledanibonites convexa Pand., Beitr. z. Geogn. Russi. p. 91, pi. 19, fig. 5. 



1845. Leptcena convexa Vern. i part.) Géol. de la Russie p. 232, pi. 15, fig. 5. 



1890. Leptaetm convexa G.vgel, Brach, d. cambr. u. sil. Gesch. p. 49, pi. III, fig. 24. 



It is very difficult to know from literature what is really meant by 

 this form, and I do not think I should have dared refer with certainty 

 any of my specimens here, had I not had before me a Russian specimen 

 (from the Kuckerszone) determined as P. convexa by F. Schmidt. With 

 this specimen, a Norwegian one from the lower part of 4 b, Langesund-Skien 

 district, exactly corresponds in size and form. The surface of the speci- 

 men is so much worn that nothing can be seen of the striae. The outline 

 is subtriangular, width 7, length 5,5 mm. The convexity is very strong, in 

 both longitudinal and transverse sections, the lateral sides sloping very 

 steeply from the median elevation, becoming flatter towards the lateral 

 margins. The umbonal portion is very prominent, projecting beyond the 

 hinge line. 



Occurrence. 4b, lower part, Skjælbugten, Langesund — Skien 

 district. 



Plectambonites qainquecostata M'Coy. 

 (PI. XV, fig. 9.) 



.Synonom3' see Leptcena quiiiquecostata M'Cov in Davidson, Brit. Sil. Brach, p. 322. 



Mat. pres. A small number of generally fragmentar}' ventral valves 

 lying in rocks. 



Description. Rather smal, outline regularly semi-circular, much 

 like in smaller specimens oï P. sericea. Convexity like or .somewhat more 

 expressed than in that form, varying. 



Radiating striæ very delicate, and all of equal size, except for a very 

 small number of stronger ribs, in the posterior portion generally only 

 three, one near the median line, and two in the lateral portions; near the 

 frontal margin the number is somewhat larger. Fine concentric raised lines 

 are seen all over the surface, especially well near the rounded margin. 



Observations. In size, form and surface characters my speci- 

 mens seem to correspond ver}' well with this well known British specimen, 

 that in Davidson's monograph is stated to occur from Upper Llandeilo 

 to Upper Llandovery, thus throughout a considerable space of time. 



Occurrence. 4b«, Holmen, Asker. 



