86 OLAF HOLTEDAHL. M.-N. Kl, 



like that in fig. 8 of Davidson's plate XLVIl. A common type is also one 

 like Davidson's fig. lo. The size also varies strongl}^ the specimen 

 depicted in pi. XVI, fig. 5 of the present work being twice as large as 

 the smaller specimens of my material. 



A very characteristic feature is the fine concentric regular lineation 

 always met with in well preserved surfaces. 



In the internal moulds we observe the same typical features as in 

 the British specimens. 



Observations. PL tenuiciiicta is certainly extremely nearly re- 

 lated to the P. oblonga Pand from the Ordovicic of the Baltic provinces 

 but cannot be united with that form, of which I have seen typical speci- 

 mens. As Davidson emphasizes, a difference exists in surface characters, 

 the P. tomicintia being distinctly concentricall}- striated, P. oblonga being 

 smooth — or in a ver}- good state of preservation, — longitudinall}^ 

 striated. 



Occurrence. The zone for which P. tciiiiiciucfa is a very typical 

 fossil is 4 b a, the fossil occurring rather commonly there in Ringerike 

 and Asker, At Bratterud, Ringerike, the form is found in what is con- 

 sidered the transition zone to 4 b /^. A single outer mould is found in 

 the Coelosphæridiuni zone of the MJosen district. 



The fossil is also found in 5 a, from Vestre Svartø, Ringerike, and 

 in 6 a (?) Spirodden, Asker, the total material from these localities being 

 however ver}- fragmentary. Yet the specimens seem to correspond well 

 with those from 4 b. 



Genus Triplecia, Hall 1859. 



Triplecia iusalaris Eichw. 



(Fl. XVI fig. 9.) 



1842. Terebratitia insitlaiis EicHW, Urwelt Russlands, Vol. II, p. 49, pi. II, fig 6. 



1845. Spirifer — Verneuil Géol. Russ.. p. 149, pi. VIII, fig. 7, 



1858. Orthis i?i — Schmidt. Unters, Sil. Form, Esthland p. 216. 



1 86 1. Spirifer — Roemer. Foss. Fauna d. Geschiebe von Sadewitz p. 45. pi. V, fig. i r. 



Of specimens which strictly speaking are identical with Eichwalds 

 Terehratiila insnlaris from the Lyckholm of Dago, only a few are to hand. 

 A fairly well preserved specimen from 4 d Frogno, depicted pi. XVI fig. 9 

 shows especially well the characteristic features of the species. In two 

 pecimens the apex of the ventral \alve is seen to be perforated by a 

 small circular foramen. 



Occurrence. 4 b (?) Ringerike, Eker, 4 d, Ringerike ; 5 a. Asker (?) 



