1915- ^O. 12. THE STROPHOMENIDAE OF THE KRISTIANIA REGION. 49 



Strophomena Bryggeri n. sp. 



(PL VII, fig. 3-5.) 



Mat. pres. Agréât number of dorsal and outer moulds of ventral 

 valves in rock. 



Description. Rather large, outline semi-circular to rounded 

 triangular. Cardinal extremities usually broken, their angle seems gene- 

 rally to be 50 - 70^. The width and length in mm. of two mature individuals 



are given below: 



Width 42 40 



Length (hinge to front) 30 28 



In the ventral valve a slight convexity near the beak, the valve at a 

 distance from the hinge of from \'o to ^/3 of total length of the shell geni- 

 culating more or less sharply, concavely, with an angle of no — 140 ^. 

 The dorsal valve follows the ventral closely, the central disc is here 

 entirely flat. 



Radiating ribs crowded, of medium strength and in the postero-lateral 

 portions of equal size, while in and near the median portion we notice a 

 series of ribs, much stronger than the rest, separated by a number (2 — 6) 

 of the ordinär}' fine ones. The stronger ribs are not equal, those near 

 the median line being decidedly stronger than the more lateral ones, that 

 graduall}' pass into the size of the finer striae. This radiating striation 

 is crossed in the central disc by a ver}' distinct irregular concentric 

 corrugation, exactly like in Leptacna rhoniboidalis. 



As to the internal characters of the species, I have succeeded in 

 removing the shell substance in one ventral and a dorsal valve; the 

 internal cast of the portion near the beak of the dorsal one, thus seen, 

 is depicted in pi. \'1I, fig. 3. In the ventral valve, the muscular area is 

 only slightly pronounced, bordering elevated ridges only being present 

 quite near the hinge, from where they are seen, diverging, for a distance 

 of about 5 mm. Probably the muscular area has been very small. In 

 the dorsal moulds we find two central elongate cavities, corresponding to 

 a distinctly bilobed cardinal process and further outwards narrow cavities 

 for the crural plates at an angle with the hinge of about 45 '^. The crural 

 plates graduallv lose in strength anteriorly. In the median plane an indi- 

 cation of an elongate depression, corresponding to a median low ridge in 

 the valve, is seen. 



Observations. In exterior characters this species comes very 

 near to the PaIaeosiroplio)nciia coticava Schmidt described above. Both 

 the general form and the surface characters are very similar in the two 



Vid.-Selsk. Skrifter. I. M.-X. Kl. 19 15. No. 12. 4 



