84 OLAF HOLTEDAHL. M.-N. Kl. 



As to the striation of P. tratisversalis, we commonly find a greater 

 distance between the major striae and a larger number of finer ones in 

 each interval than are found in P. sericea, but we may also have specimens 

 that have a surface ornamentation exactly as in that species. 



The interior characters of both ventral and dorsal valves can be 

 nicely seen in a number of internal moulds and exhibit the well known 

 features (see pi. XV, fig. 3). 



Observations. The interior, and generally also the outer 

 characters of P. traiisversalis distinguish the species well from P. sericea 

 and seem to place it near P. Sclunidti from 5. hi fact, specimens of the 

 latter may often only with difficulty be kept apart from large and relatively 

 long specimens of P. transversalis from 6. The only distinguishing external 

 characters that I know of are the somewhat greater coarseness of the 

 main radiating striae and the presence (in both valves) of somewhat more 

 strongly marked wrinkles near the hinge line on both sides of the beak 

 in the older form. 



Occurrence. Plectauibointes tra)isversalis has been found in all 

 the Siluric districts of the region, the total vertical range being from the 

 base of stage 6 to the top of 8. 



Plectambonites scissa, Salter var. triangularis n. var. 

 (PI. XV, fig. 5-6.) 



Mat. pres. A few internal moulds of ventral and external moulds 

 of dorsal valves. 



Description. Shell small, all specimens being about 7^/2 mm. 

 wide, 4 mm. long, rounded triangular in outline, ventral valve very convex, 

 dorsal valve moderately concave. Cardinal angles 50 — 60°. The cardinal 

 area of the ventral valve of moderate height, that of the dorsal valve not 

 much lower. Height of both added together about I'/o mm. 



Radiating striae can be seen near the rounded margin in one outer 

 mould of dorsal valve, their distance is about i mm. In one half of this 

 specimen 8 ribs were counted reaching to the border. Between these 

 ribs others, very delicate ones have certainly existed, but cannot be seen 

 on the worn surface. 



The ventral muscular field has been deepl}' concave, bordered by 

 high ridges, strongly sinuate in front, with a low median ridge that loses 

 in strength posteriorly. The muscular impressions have been faintly striated 

 concentrically, parallel to the frontal lobes. 



