1915- ^O- 12. THE STROPHOMEXIDÄE OF THE KRISTIANIA REGION. 85 



Observations. This shell comes excedingU' near to the British 

 P. scissa Salter^) from the Lower and Upper Llandovery, but shows some 

 minor differences that keep it apart. The outline is more triangular, both 

 cardinal angles and frontal curvature being more sharp in the Norwegian 

 specimens than in Davidson's figures. Furthermore the surface of the 

 British species is stated b}* Davidson to be »ornamented witli four or five 

 slender, thread-like, rounded radii, with broad interspaces finely and 

 longitudinally striated-, whilst the number of radiating main ribs in the 

 Norwegian form is much larger, the striation of the latter being rather 

 different from that of Plcctambomtfs quiuquccostata, which P. >cissa accord- 

 ing to Davidson is very like in surface ornamentation. The P. scissa 

 and its Norwegian relative have quite peculiar internal features, differing 

 somewhat from that of P. scricca and P. transversalts. Both the ventral 

 valve, with its ver3- strong ridges surrounding the muscular field, and the 

 dorsal one (see Davidson 's illustration loc. cit. fig. 25) show differing char- 

 acters, those of the ventral valve reminding one somewhat of Strophotntna 

 or Strophonella. 



Occurrence 6 c. Railroad section near the Asker Farm. Asker. 



Genus Christiania Hall and Clarke 1892. 



Christiania tenuicincta M"Cov. 

 (PI. XVI. fig. 4—81. 



1848. Producta tenuicmcta M'Cov. Sil. Foss., Ireland, p. 25, pi. Ill, fig. 4. 



1852. Leptæna — • — M'Cov. Brit. Pal. Foss., p. 239, pi. I H, fig. 40 



1859. — »— — »— S.\LTER. Siluria, 3rd. ed.. p. aio, fig. 4. 



1866. — » — — >— S.^LTERMem. Geol. Sur\ey, vol. Ill, p. 267 



1871. — »— — .— D.wiDsoN Brit. Sil. Foss., p. 326, pi. XLVII. fig. 7—18. 



A rather large number of free specimens of this interesting little form 

 are present, many of which, where the shell substance has been naturally 

 or artificially removed, reveal also interior characters. 



Like the British specimens, the Norwegian ones show much variation in 

 outline, most of them being rather elongate and oval. We generally ob- 

 tain the impression that the width of the shells at the hinge line is dis- 

 tinctly inferior to the width further towards the front, but in most cases 

 this is certainly due to the tact that the cardinal angles are broken. In 

 outer moulds of one or two dorsal valves the impression of the cardinal 

 angles could be exactiv seen and the outline was there found to be much 



1 See Davidson. Brit. Sil. Brach, p. 325, pL XI. VII, fig. 21—2 = 



