IA OLAF HOLTEDAHL. M.-N. Kl. 



Genus Bafinesquina Hall and Clarke 1892. 

 Rafinesquina (?) sp. 



Strophomena imbrex Pander, var. confr. seniiglobosa Dav. Brøgger, Die silurischen 

 Etagen 2 und 3, p. 50, pi. XI, fig. 6. 



As the material of this species is extremely poor, it is not possible 

 to obtain any exact idea as to the general appearance of this form. The 

 most complete fragment, that probably presents a ventral valve, shows a 

 faint, rather regular convexity. However, as the characters of the um- 

 bonal region cannot be seen, we do not know with certainty that it is 

 the ventral valve, in other words that we are dealing with a Rafinesquina 

 and not with a Strophomena. 



The surface characters can be well studied. The ornamentation con- 

 sists of a series of distinct, regular ribs, between two of which we find 

 from 3 to 6 much finer raised lines. 



Proceeding towards the frontal and lateral margins the number of the 

 stronger ribs greatly increases, one of the minor lines in the middle of 



the interspaces between two stronger 

 gradually attaining the coarseness of 

 the latter. Crossing the radiating ribs 

 occur a series of crowded concentric 

 raised lines, the distinctness of which 

 varies considerably in different frag- 



/?fl;Ç«fsr/»/;w (?) sp. with surface ornamentation. mentS. The drawing in BrøGGEr's 



Reprinted from Brøgger, Die sil. Et. 2 u. 3. paper, here reproduced, gives a good 

 picture of the surface characters at a spot where the concentric lines are 

 well developed. 



Observations. As Brøgger especially emphasizes, this form 

 with its slight convexity differs very greatly from the typical Stropho- 

 mena (Rafinesquina) imbrex Pand. It is certainly also distinctly distin- 

 guishable from Davidson's species of the high Siluric, Strophomena (Stro- 

 pheodonta) semiglobosa. If it is the ventral valve that is convex, the 

 nearest relative of the form is the Strophomena (Rafinesquina) Nanseni 

 Kiær described from Ordovicic beds (calcareous slates) of Khabarova on 

 the south side of Yugar Strait ^. Prof. Kiær also mentions here the pos- 

 sible relationship between that form and the one figured by Brøgger. 

 The age of the Khabarova rock also seems to correspond fairly well with 

 the stratigraphie horizon of the Norwegian form. 



1 See Kiæk: The Lower Silurian at Khabarova. The Norwegian N^rth Polar Expedition 

 1893 — 96. Scientific Results Vol. IV 1900 — 1904. 



