44 



OLAF HOLTEDAHL. M.-N. Kl. 



This process has certainly not been bilobed, but was a simple, vertical, 

 narrow plate, as in orthoid brachiopods. The length of the plate in the 

 larger valve is 2,5, in the smaller 1,5—2, the plate gradually becoming lower 

 and less prominent anteriorly. On studying the best preserved mould in 

 good light, we observe in the inner part of the elevated ridges, bordering 

 the median furrow, a faint longitudinal depression that possibly may be 

 understood as minute lateral extremities of the cardinal process, the latter 

 being in this case slightly trilobate. The process has been supported 

 by a low well-rounded, median ridge continuing for a short distance 

 anteriorly. 



The crural plates diverge anteriorly at an angle of a little less than 

 90°, increasing in strength anteriorly and ending very abruptly after a 

 length of about 2,5 mm. 



Observations. It is beyond doubt that the interior of these 

 dorsal valves shows stronger relations to an Ortliis than to a Strophornena 

 whilst at the same time the exterior, in size, outline and surface characters 

 is as typically Strophomenoid as possible. It is of course of the greatest 

 interest that we have the characters of these two related families united 

 in one species, and furthermore that this species occurs among the oldest 

 Strophomenids known. 



The species is identical with a Baltic form (from the Vaginate-lime- 

 stone), of which I have seen a large number of specimens. It is certainly 

 this form for which Schmidt in 1858 proposed the name Strophornena con- 

 cava, giving a description but no illustration of the species. This form 

 is according to the diagnosis given by Schmidt concavely geniculated 

 (»die Schaale nach der Ventralseite zu gekniet«) with flat circumbonal 

 portions (»beide Schaalen bis zum Knie fast flach«). As to the surface 

 characters Schmidt writes: »Oberfläche mit schwachen Längsrippen, die 

 sich nach dem Stirnrande zu vermehren, und starken unregelmäßigen 

 Querrunzeln (5 — 6 bis zum Knie) bedeckt«. 



Occurrence. "^ c ß, Huk, near Kristiania, 



3 c ^, (lower part), Vestfossen, Eker. 



Genus Strophornena, (Rafinesque) De Blainville 1825. 



Hall and Clarke in their Introduction summarised the more important 

 part of the history of Rafinesque's term Strophomenes. The name 

 was by Blainville used for the American lorm, Strophornena rugosa, 

 a form with concave ventral and convex dorsal valve. King, and later 

 on Davidson, included under Strophornena also forms with convex ventral 



