1915- No. 12. THE STROPHOMENFDAE OF THE KRISTIANIA REGION. 45 



and concave dorsal valves (like S. altcruata Emmons), or nearly flat valves, 

 (like S. peden Linn.j. Hall and Clarke then restricted the range of the 

 genus Strophomcna to forms with the resupinate valves of Strophomena 

 nigosa Blainville and smooth, not denticulated cardinal areas. 



Strophomena (?) Nefedjewi Eichw. 

 (PI. IX. fig. I.) 



i860. Leptœna Nefedjewi Eichwald, Lethæa Rossica p. 859, pi. XXXVI, fig. 13 a — d. 



Mat. pres. Two fragmentar}' dorsal valves in rock. 



Description. Shell of small size, semi-elliptical outline, broadly 

 rounded in front, greatest width at hinge. Cardinal angles (one fairl}' well 

 preserved) 70 — 80°. The total width has been 15 mm., length 9 mm. Dorsal 

 valve slightly convex, nearh" flat, greatest convexity in the marginal portion. 

 Surface markings consist of a system of inconspicuous radiating ribs, the 

 number of which at the lateral margin in one half of the shell amounts to 

 not more than about 15. Between these striae no trace of other and finer 

 ribs, as common in younger Strophomenids, is seen. The intervals, as 

 well as the radiating ribs, however, are crossed b}' very fine, crowded, 

 concentric, raised lines. At various places more marked concentric lines 

 of growth are present. 



Observations. The occurrence of this little primitive Stropho- 

 menid, very common in zone B III of the Baltic provinces, is of conside- 

 rable interest. As Lamansky states in ^T>\e ältesten silurischen Schichten 

 Rußlands« ^, p. 159. the species is certainU' closeh' related to his genus 

 Plectella of still older beds. Of younger forms I know of none coming 

 near to it. I doubt very much whether 5. Xcfedjeivi can be included in 

 the genus Strophomena, but as the interior characters are not seen in the 

 Norwegian specimens, the question cannot 3'et be decided. 



Occurrence. 3c/:?, Toien in Kristiania; Vestfossen, Eker. 



Strophomena I?) hemispherica, F. Schmidt. 

 (PI. VI, fig. 10.) 



Mat. pres. A single outer mould of a ventral valve. 



Description, Shell of rather large size, rounded triangular in 

 outline, considerably deformed by pressure. Greatest width at the hinge line. 

 Exact character of cardinal extremities cannot be seen. The valve has 

 been slightly- convex in the umbonal region, then slightly concave, from 

 about the middle of the length of the valve strongl}' deflected, as ma^' be 



1 Mém. Com. Géol. X. S. B. 20. St. Petersburg 1905. 



