I9I5- ^O- 12. THE STROPHOMEXIDAE OF THE KRISTIANIA REGIO.V. 47 



inferior to the median length of the central disc, but this may only be due 

 to the front being broken. The dorsal valve in the circumbonal portion, 

 that is the only one preserved, has a distinct, broad, but shallow longi- 

 tudinal depression (sinus). 



The surface markings consist of very fine, radiating, raised lines, 

 some of these (the inter\als being very irregular) being slightly stronger 

 than the rest. In the central disc are further seen a number of rather 

 faint, yet distinctly visible concentric wrinkles, strongest near the hinge 

 which the}- meet at a very sharp angle. 



Observations. The occurrence of this little form in the Kristi- 

 ania region is of considerable interest, as it certainly is a good guide 

 fossil, previously being known from corresponding zones of Western 

 Russia and Sweden. Lamaxsky (1. c.) in a note i6, p. 177, thinks the 

 Strophomena rhomboidalis var. that Brøgger has mentioned and depicted in 

 >Die sil. Et. 2 u. 3' to be the Strophomena Jentzschi; as will be seen in 

 this paper, p. 43, it is another and different form. 



Occurrence. One species is labelled 3 c 3 — 3 c /, o: the transition 

 zone between these two horizons. Vækkere, near Kristiania, the other 3 c, 

 Teicn in Kristiania. 



Strophomena pseadodeltoidea Stollev. 

 (PI VI. fig. 2-3.1 



1895. Strophomena pseudodeltoidea Stollèy. Die cambr. u. sil. Geschiebe Schleswig-Holsteins 



u. ihre Brachioptodenfauna. I. p. 41. 

 1907. Strophomena pseudodeltoidea WntAX. Studien über das Nordbaltische Silurgebiet II, 



P- 145- 



Mat. pre 5. A fairh" large number of dorsal and moulds of ventral 

 valves in rock. 



Description. Shell of medium or rather small size, outline (as 

 seen from above) sub-semicircular, nicely rounded in front, with projecting, 

 rather sharp, cardinal angles. \'entral valve slightly convex in the um- 

 bonal region, then slighth" concave with antero- lateral margin strongly 

 deflected. Dorsal valve flat in central disc, margins mostly abruptly de- 

 flected at about right angle. Length of deflected portion in median line 

 generally considerably exceeding length of central disc. Frontal part of 

 the marginal rim often slightly nasute, fold-like. 



Surface with straight radiating delicate ribs of two distinct sizes. 

 Major ribs with varying, relatively very broad intervals, the number of 

 intervening finer ribs not seldom as much as 10 — 15. In places very 

 fine, crowded, waving, concentric, raised lines are observed. One very 



