I9I5- No. 12. THE STROPHOMEMDAE OF THE KRISTIANIA REGION. 21 



Eafinesquina pseudoalternat a F. Schmidt. 

 (PI. Ill, fig. 9— II.) 



1858. Strophomnut pseudoalternata F. Schmidt. L'niersuchungen über die Silurformation 

 von Ehstland etc., p. a 16. 



Mat. pres. About a dozen specimens, partly ventral, parth' outer 

 moulds of dorsal valves. 



Description. Shell of medium and fairly constant size, two 

 specimens measured having a greatest width : 27 and 22 mm. : with 

 corresponding length: 21 and 18^0 mm. Outline semi-elliptical, well 

 rounded, with cardinal angles at about 90° or slightly less, except for a 

 small projecting wing at the very hinge line. In a single specimen (dorsal 

 valve) the shell widens in front of the hinge, the greatest w'idth being at 

 some distance from the latter. Convexity moderate, very slight in the 

 posterior and central part, where however the beak, that projects beyond 

 the hinge-line, is elevated from the surrounding part of the shell; rather 

 strongly in the zone, bordering to the lateral and anterior margin. We may 

 have a relatively abrupt deflection of this margin or a more gentle one. 

 The dorsal valves have a flat or very faintly concave central disc, and 

 then a more strongly deflected marginal rim, but with no sharp bordering 

 line between the two. Length of the deflected margin in the median line 

 amounts to about ^3 of the total length of the shell, sometimes more, 

 sometimes less. 



Surface markings consist of radiating ribs of two distinct sizes, rather 

 regularly arranged. Intervals between the stronger ribs i — 2 mm. in the 

 marginal portion of the shell, number of intervening striæ 4 — 8. On well 

 preserved parts of the surface fine and crowded, somewhat undulating con- 

 centric lines can be seen. The most characteristic feature is the constant 

 presence of a small number of short but distinct corrugations stretching 

 from the hinge line and some millimeters, in some cases as much as 

 ^ cm., nearly vertically away from it. The corrugations are most marked 

 about midwa}- between beak and cardinal extremity. 



In the ventral valves present there is an indication of regular con- 

 tinuous concentric corrugations or depressions in the surface also in the 

 rest of the shell near the latero-frontal borders. 



Of interior characters 1 know ver}' little. The ventral muscular area 

 seems to be fairly large; the width in one specimen found to be nearly Vj 

 of the width of the shell. 



Observations. That the species above described is identical 

 with the form that Schmidt (1. c.) described as Strophometia pseudo- 



