1915- No. 12. THE STROPHOMENIDAE OF THE KRISTIANIA REGION. 15 



Even if we assume the latter to be a true Rafinesqttina, there will be 

 differences between the two forms, Rafifiesqiiina Xanseni being relatively 

 longer and narrower, and furthermore possessing distinct wrinkles near 

 the hinge-line. 



As the detailed characters of this form are not known, I will refrain 

 from giving it any specific name. 



Occurrence. 3c. j and 3c/, Vestfossen, Eker and near Kristiania. 



Rafinesquina imbrex Pand. 

 (PI. I, fig. 8. 10. II.) 



1830. Plectanibonites imbrex, PI. triangularis Pand. Beitrage zur Geognosie d. russischen 



Reiches, p. 91, pi. XIX, fig. 12, ir. 

 1845. Leptaua imbrtx Verx. Géologie de la Russie d'Europe, vol. II, p 230, pi. 15, fig- 3. 

 1890. Strophonuna inibrtx Gagel. Brach, d. cambr. u. sil. Gesch., p. 40, pi. III, fig 14. 



Material present. A few fragmentary specimens and an interior 

 mould of a dorsal valve is all that can be identified of this form, so 

 extremely common in the middle Ordovicic of the Baltic Provinces. 



Description. None of the specimens at hand are sufficiently 

 complete to give an exact idea of the outer characters of the shell. Size 

 varying considerably, width at hinge line in one specimen 18, in another 

 15 mm. In the best preserved specimen it is evident that the width at hinge 

 line does not represent the greatest width, the shell for some distance 

 widening distinctly towards the front. The convexity is different in dift'e- 

 rent specimens, no very sharp geniculation is ever seen, the most pro- 

 nounced curving of the shell being, however, found at a distance from 

 the hinge of about ^ 3 of the total length of the shell. This portion 

 nearest to the hinge, representing the central disc, is in two specimens 

 only slight!}- convex, in a third one much more so. In the largest speci- 

 men found the cardinal areas can well be seen, that of the ventral valve 

 being relatively high, nea-^ly rectangular in outline and with a fairly distinct 

 longitudinal lineation. A smaller specimen shows a less high ventral area, 

 the outer line of it also sloping more laterally. 



The surface in all specimens is greatly worn, but in some places we 

 see a system of distinct, radiating ribs, with a distance between each of 

 about I mm. and in the interspaces indications of the presence of minor 

 ribs are visible. Also traces of fine concentric lines can be seen in a 

 good light. 



In the mould of the interior of the dorsal valve, depicted in pi. I, 

 fig. 8, two distinct grooves, corresponding to ridges in the valve, can 

 ver}- plainly be seen on both sides of the median line. 



