28 OLAF HOLTEDAHL. M.-N. Kl. 



specimens. This différence is seen in the outline of the posterior part of 

 the ventral muscular area, the posterior angle of that field being con- 

 stantly, in all of Davidson's figures, more obtuse than in any Norwegian 

 specimens studied. This feature then causes a somewhat broader outline 

 of the elevated umbonal region also in the surface. The angle of the 

 muscular field in Davidson's illustrations varies from 90 ^ to more than 

 100*^; in the Norwegian material from 80 '^ down to 45°. The muscular 

 field according to the illustrations also are more distinctly limited anteriorly, 

 but this difference may be due to a different state of preservation. 



Rafinesquina expansa is without doubt nearly related to the American 

 R. alternata Em., but the two species generally can easily distinguished 

 by a different convexity, the American species having the greatest con- 

 vexity near the latero-anterior border in typical specimens, while in 

 R. expansa the strongest curving is found in the umbonal region. As to 

 interior characters R. alternata has relatively much larger muscular scars 

 in the ventral valve than the other. Another exceedingly nearly related 

 American form is Brachyprion (?) corrugata Conrad from the Clinton. 



As to the generic position of this form, I have not in my material 

 been able to study the cardinal area as to the presence of denticulation 

 but in the figures given by Davidson no indication of any is shown, 

 proving the species to be a Rafinesquina and not a Stropheodonta. It is 

 of interest to mention than in a form from the Lower Siluric of the Anti- 

 costi Island, Canada, which undoubtedly comes near the R. expansa, being 

 however considerably smaller, viz. Brachyprion leda Billings, the cardinal 

 area of the ventral valve shows an initial denticulation. 



In the paper of C. Gagel »Die Brachiopoden der cambrischen und 

 silurischen Geschiebe im Diluvium der Provinzen Ost- und Westpreufeen« ^ 

 (p. 43, pl. V, fig. 24) is described a form, given a new name Strophomena 

 Ponipecki, that according to description and illustration does not seem to 

 be different from specimens of R. expansa. 



Occurrence. Rafinesquina expansa is found in the Mjesen 

 district. Hadeland, Ringerike, Asker — Bærum, Skien — Langesund district, 

 with a total vertical range from 5 b to 7 a, especially common in the large 

 typical specimens, in 6. 



* Beiträge zur Naturkunde Preußens. 6. Königsberg 1890. 



