1915- ^'^- i- THE STRöPHOilEXiDAE OF THE KRISTIANIA REGION. 51 



Strophomena Asmasi \'er.n. 

 (PI. Vm, fig. 1—8. pi. IX, fig. 2). 



1845. Ortlus Asntusi Verxectl, Gêolofiie de la Russie. VoL II, p.' 191. pi. X, ßg. 17. 



i86£. Ortkis Asmasi Roexer. Fauna der Geschiebe von Sadewitz, p. 50. 



? 1880. StrcfAomtemm bam Tørnocist. Axgeixs-Lindstrox, Fragmumtm sAtrkm p. 29, pL XTV, 



fig- 43- 

 18&4. Stro p lt mm t M m Asmusi KiESOW, SiL n dev. Geschiebe Westpreuêens. p. 40. 

 1890. S tio/ Âo u i eM m Asmmsi GAtsi^ Brach, d. cambr. a. sU. Geschiebe, p. 46, pL III, fig. 13. 



Strophometia Astmtsi (and nearly related forms) belong to the Stro- 

 phonienids most commonly met with in the younger Ordovicic rocks of 

 the Kristiania region. When studying this form it verj* soon became 

 evident that it was a highly varying one, different zones and also different 

 districts generally not containing homogeneous t3"pes. It wül be —.cs: 

 natural to deal with these different types separately. 



4 b. RÎDgerike. (PL VIU, fig. i— 3.) 



Ma:, pres. A large number of ventral and dorsal valves in rock. 

 Size :"a:r!y large, outline broad, nearly semicircular, broadly rounded in 

 front, lateral margins gradually becoming more parallel posteriorly till 

 reaching the hinge line at an angle verj' slighdy less than 90**. The 

 dimensions in mm. of a number of specimens are given be!ow: 



Width 27 31 39 43 



Length 18 21 23 25 



Ventral valve slightly convex for a small portion of the surface near 

 the beak, otherwise \ ery slightly concave. Dorsal valve slightly convex, 

 except for a faind}' expressed longitudinal depression in the median line 

 from the hinge and half-way towards the front. 



Surface marking very pretty and regular, with a series of major 

 radiating ribs at comparatively large intervals (on an average for the 

 antero-lateral portions of the surface about 2 mm.) with 5 — 10 finer, though 

 easily seen ribs between each pair. Towards the frontal and lateral 

 margin one of the finer ones in the middle of the interspace increases 

 in size and gradually becomes as strong as the rest of the major ribs. \'ery 

 fine, crowded, concentric lines are visible b\' the help of a magnifying 

 glass. -A. few lines of growth may be seen, especially near the rounded 

 margin- Of concentric wrinkles there is no trace. 



The shell substance that is very thin, was removed by acid in a few 

 specimens, and the interior characters were laid open in moulds (see pi. VIII, 

 fig. 2 — 3). The ventral muscular field has been strongl}- marked for a short 

 distance in the posterior portion, the bordering ridges having ended rather 



