8o OLAF HOLTEDAHL. M.-N. Kl. 



Observations. This form comes very near to the Swedish 

 PL trabeata from the Trinucleus division ^ in which also the same con- 

 centric surface ornamentation is found. According to the figures given by 

 Angelin-Lindstrøm the cardinal angles in the Swedish form, however, are 

 only slightly less than 90°, while in the Norwegian form they are always 

 very sharp, the outline of the two forms thus being different. 



It is of interest here that this form, though coming in surface 

 features nearer the PL transversalis — and PL Schmidti type than the PL 

 sericea type in interior characters shows a near relationship to the latter. 



Occurrence. 4b, City of Kristiania. 



Plectambonites sp. 

 (PI. XIV, fig. 8.) 



A few ventral valves in rock strongly remind us of the specimens 

 just described, the concentric surface markings are, however, less distinct. 

 They are besides distinguished by the somewhat undulating surface of the 

 shell near the frontal border. 



Occurrence. 4b a. Delebugten, Asker. 



Plectambonites sericea, Sow. var. rliombica M'Cov. 

 (PL XIV, fig. 4-5.) 



1852. Leptœna sericea Sow. var. rhonibicn M'Coy, Brit. Pal. Foss., p. 239. 



187 1. Leptœna sericea Sow. var. rhonibica Davidson, Brit. Sil. Brach., p. 325, pi. 48, 



fig. 20—32. 

 1884. — » — — s — — » — Suppl., p. 169, pi. XII, fig. 4 — -7. 



A large number of specimens in rock are present. Outline much as 

 in P. sericea Sow., but the specimens are of considerably larger size 

 than normal individuals of that form; a specimen of medium size having 

 a width of about 20 mm., a length of about 13 mm. The convexity is gene- 

 rally more expressed than in the ordinary P. sericea, however not greatly 

 so. An average ventral valve of the size just mentioned shows a con- 

 vexity (height) of 4 — 5 mm. The striation is quite like that in P. sericea, 

 with a large number of fairly distinct radiating ribs, between two of which 

 generally come from 2 to 7 very delicate ones. No corrugations are 

 visible, but there are sometimes faint lines of growth. 



This form, that certainly comes extremely near to the typical P. sericea, 

 corresponds very closely with Davidsons figures of Leptaena sericea, var. 

 rhombica, M'Coys original description not being apt to give any good 

 picture of the form. 



' See Angelin-Lindstrøm, Fragmenta silurica, 1880, p. 30, pi. XVII, figs. 1—4. 



