96 OLAF HOLTEDAHL. M.-N. Kl. 



As to the Plectambonites forms they become ver}' common from 4 b 

 and upwards. As I do not know the interior characters of the interesting 

 form from 3 c ß, Pled. (?^ Kristianiae, I have no certain premises for any 

 conclusion as to the relationship of the younger forms to this old one. 



While the bulk of the Plectambonites forms have a fairly similar 

 striation with ver}' numerous and fine radiating ribs of two distinct sizes, 

 a single one, the PL cf. papulosa Cowp. R., has nearly equal and much 

 coarser striae giving this form a peculiar aspect. I think it very probable 

 that this coarsely striated type marks a certain line in the development 

 of the Plectambonites group. The fact that this type is not found above 

 the Ordovicic and furthermore its comparatively great likeness to the 

 species from 3 c ^, points in the direction that this is a more primitive 

 type than the other, common Plectambonites forms. As to the latter, we 

 have several distincly different types of interiors : the sericea-iy^ç., being 

 the dominating one in the upper part of 4 and in 5, the transversalis- 

 type, with very strong subparallel ridges in the dorsal valve, in 6 — 8, 

 being found, however, as it seems already in 5 i^and possibly in 4) in 

 PL Schmidti, and the scissa-iy\ie. with ver}- strongly marked ventral 

 muscular scars, in 6. 



Very stabile types, much less varying than Plectainboiiites are the 

 genera Christiania, Triplecia and Streptis as far as the}- are yet 

 known. The genus Christiania is cited from B III of Russia (C. oblonga), 

 occurs in Norway as a very similer form, C. te)iitici)icta M'Coy, from 4 b 

 and to the base of the Siluric and is finally found in the Lower Helder- 

 berg of North America as a strongly resembling form C. subqnadrata 

 Hall. Triplecia insularis Eichiv. from 4 and 5 is found tliroughout 6 in 

 a variety that only by larger size and more expressed sinus and fold can 

 be separated from the older type. 



When we survey the generic or subgeneric terms used in the 

 systematic description of this paper, we find that a very large percent 

 have their origin in America. 



We have : Rafmcsquina Hall and Clarke, Brachyprion Shaler, 

 Stropheodonta Hall. Aniphistrophia Hall and Clarke, StrophoneUa Hall, 

 Schuchertella Girty, Christiania Hall and Clarke, Triplecia Hall. These 

 names have been created during a large space of time the oldest one 

 Stropheodonta in 1852, the newest Schuchertella in 1904. 



As some of these terms have not yet been fully acknowledged or 

 come into common use in European paleontological literature it might be 



