1915- No. 12. THE STROPHOMENroAE OF THE KRISTIANIA REGION. 35 



BrachyprioD philomelam Billings. 

 (PI. V, fig. 1-3.) 



i860. Strophomma Philomela Billings. Canadian Naturalist and Geologist. Vol. V, p. 56. 

 1865. Strophomena Philortula Bill. Palaeozoic Fossils p. 122, figs. 100— loi. 



A small number of ventral valves and a single outer mould of a 

 dorsal valve are all that can be referred to this species. 



These specimens show mutually a considerable variation in general 

 form, and must therefore be shortly mentioned separately. 



The two specimens in one slab depicted in pi. V, fig. i, the longer 

 a ventral, the shorter an outer mould of a dorsal valve, exhibit in spite 

 of some difference in outline a common tvpe, being broadl}' rounded 

 anteriorly, broadening towards the hinge, where they run out into rather 

 sharp cardinal angles. The convexity is somewhat strong. 



Radiating ribs of two sizes, the intervals between the stronger ones, 

 that are rather fine, are narrow, especially so in the longer specimen, where 

 only 2 — 3 finer ribs ordinarih' occur in each interspace. In the shorter 

 specimen (dorsal), besides the ordinary, crowded and very fine concentric 

 lines, we see an indication of concentric wrinkles. 



Together with these two specimens is found another ventral valve 

 depicted pi. \', fig. 3, coming between the two others in outline, like 

 them in convexity and striation. Traces of wrinkles at the hinge are 

 possibl}- present, too faint to be distinctly seen. Already these three 

 specimens, from 7 b, Limaastangen, Ringerike, show that the form under 

 consideration shows extremely great individual variation, and this impres- 

 sion is furthermore strengthened b}' studying a slab from a slightly lower 

 horizon, 7 a, Bjerke off Holmestrand. Here two ventral valves are present, 

 one depicted pi. V, fig. 2 is decidl}' triangular in outline (in outer form very 

 much like a reversed Stropliomlla cuglyplta), the other much rounder (the 

 exact outline cannot be seen owing to an inferior state of preservation). 

 Yet they certainK- belong together with the three others to one var3-ing 

 species. To judge from Billing's illustrations a good deal of variation 

 is found also in the Canadian specimens of Brachypriou philotitelum. 



A specimen that is very much like those mentioned above, but which 

 is found in much younger beds, viz. in cd (in Lange off Holmestrand) 

 is depicted pi. Y, fig. 4. 



The specimens referred to Brachypriou philomelum are without doubt 

 neariv related to the form described below, but difler by more triangular 

 outline, with relatively wider hinge. 



Occurrence. 7, Limaastangen, Ringerike (7b); Bjerko off Holme- 

 strand (7 al; 9 d. Langø off Holmestrand (?). 



