1 915. No. 12. THE STROPHOMEXIDAE OF THE KRISTIANIA REGION. 39 



Brachyprion Walmstedti Lindström. 

 (PI. V. fig. 6-8.) 



i86o Strophomena Walmstedti Lixdström. Kgl. «venska vet. ak. handlingar r86o, p. 372, 



pi. XIII, fig 16. 

 1868. Strophonietia Walmstedtt D.widson, Trans. Geol. Soc. Glasgow. Pal. Ser., vol. I, 



p. 18, pi. III. fig. 5 — 6. 

 1871. Siroplioiiuna U'alnisttdti T>.w , Brit. Sil. Brachiop., p. 290, pi. XL, fig. 6 — 8. 



Mat. pres. A fairly large number of specimens, mostly ventrai 

 or moulds of dorsal valves in rock, the majority very fragmentary. 



Description. Size rather large, general form varying greatly. 

 The figures below give width and length of a number of specimens in mm. 

 As the cardinal angles are rather sharp the figures for width at hinge line 

 can generally not be given with full certainty. To convey a more definite 

 idea as to ratio, width — length, 1 have also given the width about i cm. in 

 front of the hinge. 



Width at hinge line 40 



— I cm. in front of hinge line 33 

 Length 38 



Also the convexity varies greatly in difterent specimens. Not only 

 the degree of curvature varies, but different specimens show a widely 

 differing form of convexity. On one side we have the well-rounded 

 globose type, giving a relatively similar section in both longitudinal and 

 transverse directions. By transitional specimens we then reach the other 

 tj'pe, having a longitudinal median keel-like elevation in the ventral, a 

 depression in the dorsal valve, more pronounced, however, in the ventral 

 one. This fold and sinus is especially conspicuous near the front. It may 

 be very prominent there, and yet not developed in the umbonal region. 



Radiating ribs of two distinct sizes on the surface, the intervals 

 between two stronger ones being relatively narrow with only a few of 

 the finer lines in each — often only one. A very typical feature in 

 surface ornamentation i^ the conspicuous concentric corrugations, running 

 rather regularly and continuously Towards the antero-lateral margins the 

 wrinkles gradually disappear. 



The inner margin of the ventral cardinal area has been exposed 

 for study in two specimens and was found to be denticulated, as it seems 

 for half the hinge-line, the portion nearest the cardinal extremities being 

 broken 



The ventral muscular field is well marked posteriorly as a triangular 

 depression (angle in one specimen about 55 ") ; no distinct anterior margin 

 was visible. Two elongate adductor scars can be fainty seen, the posterior 



