(ii) 54 Palssontologia Sinica Ser. B. 



also similar to Orthoceras regulare Schlotheim' and Orthoceras politum M'Coy^ , but dif- 

 fers from them in having a much slower rate of tapering and much longer septal inter- 

 vals, It may be also distinguished from Orthoceras scabridum AngeUn3 and Orthoceras 

 sodale Barrande^ by the more gentle tapering and the siphuncle being not central in 

 position. 



Horizon and Locality: Found in the upper Neichiashan formation near Sin-tan. 



Orthoceras thyrsus? Barrande 

 Plate VII., Figs. 3a-b. 



1870. Orthoceras thyrsus Barrande: Syst. Sil. de la Boheme, Vol. II, Texte III, 1874, p. 555, pi. 

 405, figs. 15-18. 



A fragment has been obtained with only three air chambers preserved. The 

 transverse section of the shell is circular. The rate of expansion is very gentle, measur- 

 ing about I in 20. 



The septa are provided with septal necks, which extend backwards to a very 

 short distance. The depth of the septa is apparently about 1/2 that of the camerae. The 

 distance between the septa is approximately equivalent to 4/5 the diameter of the conch. 



The siphuncle occupies the position between the periphery and the center of the 

 shell, being nearer to the latter than to the former. It is circular in section, having a 

 size of about i/io the diameter. 



The test is partly preserved. Its surface seems to be covered by an ornamenta- 

 tion of oblique strife, which, however, are not very clearly shown. 



Comparison: This specimen may be related to Orthoceras pleurotomum 

 Barrandes , but differs from it in the possession of a circular shell, in a different 

 character of surface markings, and in some other points. The only species which can 

 be identified with the present specimen is Orthoceras thyrsus Barrande. But Barrande's 

 species was collected from the Silurian rocks, while our shell makes its appearance in 

 the upper Neichiashan formation of Middle Ordovician age. Therefore, if this species did 

 not originate in pre-Silurian time, then this correlation may be wrong. 



Horizon and Locality: Upper division of Neichiashan formation near Sin-tan. 



Foord: Catalogue of Fossil Cephalopoda, Pt. I, p. 5. 



Foord: Catalogue of Fossil Cephalopoda, Pt. I, p. 7. 



Angelin-Lindstrom: Fragmenta Silurica, p. 4, t. IV, figs. 6-9; t. VII, figs. 8-10. 



Barrande: Syst. Sil. de la BohSme, Vol. II, Texte III, 1874, p. 453, pi. 417, figs. g-21. 



Barrande: Syst. Sil, de la BohSme, Vol. II, Texte III, 1874, p. 412, pi. 224, pi. 296, pi. 366. 



