82 OLAF HOLTEDAHL. M.-N. KI. 



form, may be taken the specimen figured in pi. XV, fig. lo. For tliis type 

 the following diagnosis may be given: outHne semi-circular, width twice 

 the length, cardinal angles rather sharp. Convexity strong, especially so 

 near the beak. Radiating striae of two distinct sizes, with 5 — 10 (some- 

 times also less than 5) finer ribs between two stronger ones. Faint con- 

 centric wrinkles generally present near the hinge. 



This form is distinguished from Pi. Scliniidti by smaller size, from 

 PL sericea by greater convexity and larger intervals between the major 

 striae, and from PL trabeata var. acuminata by greater convexity and less 

 development of concentric wrinkles. 



Occurrence. 4b (4b« — 4b/!?) Bratterud, Ringerike, 4b, Mjosen 

 district and Bundefjord, 4 c Hadeland (?). 



Plectambonites Schmidti Törnquist. 

 (PI. XV, fig. 12—14.) 



1880. Leptaena Schmidti Angelin and Ltndstkö.m, Fragmenta silurica, p. 29, pl. 14 



figs. 25 — 26. 

 1890. l.cptaena Schuiidti Gagel., Brach, d. canibr. u. Sil. ( lesch. p. 50, pl. 5, tig. 28. 



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

 also a number of fragmentary free specimens with both valves. 



Description. A large form — for a Plcctaniboiiitcs — , a medium 

 specimen measuring about 27 mm. in width, 18 in length. Outline subsemi- 

 circular, rather long, wàth cardinal angles 70—90°, varying. Convexity of 

 ventral valve generally very strong, ventral valve with length 19 mm. 

 showing a convexity (height) of 7 — 8. The figure may also be only 5 mm. 

 The convexity is strongest in the umbonal portion, this reaching far 

 beyond the hinge when seen from above. Dorsal valve following near 

 to the ventral. 



Surface ornamentation with a number of very prominent, elevated 

 radiating ribs, between two of which about 10 fine ones, however easily 

 visible without looking very closely at the shell. Towards the front, one 

 of the finer ribs in the median portion of each interspace suddenly becomes 

 nearly as strong as the striae first mentioned, not however, attaining quite 

 the same size. The stronger ribs are at the front more crowded than 

 in the posterior half of the shell, the number of fine ribs in the inter- 

 spaces being here often not more than 5. A concentric wrinkling is 

 generally seen quite near the hinge, mostly very faint. 



Besides the typical large specimen, we find in the material from 5 

 many of somewhat differing appearance, but that no doubt have to be 



