96 BULLETIN 164, UNITED STATES NATIONAL MUSEUM 



camerae are also filled almost completely with calcareous matrix, 

 leaving only narrow spaces near the outer side of the connecting 

 ring and the middle part of each camera. The outer surface of 

 the shell and living chamber are unknown. 



Cmnparlsons. — This species may be closely related to Armenoceras 

 marginale^ but the former differs in having a deeper concavity of the 

 septa, a greater septal distance, a comparatively wider siphuncle, 

 and endosiphuncular passage. 



Formation and, locality. — Middle Ordovician, Ssuyen formation: 

 Lower fossil horizon of the black banded limestone beds, just north 

 of the Pen-hsi-hu colliery, Manchuria. 



^oZoJSy^e.— U.S.N.M. No. 83731. 



ARMENOCERAS RICHTHOFENI (Freeh) 

 PlATE 20, FiGUBE 10 



1911. Aciinoceras richthofeni Fkech, in von Richthofen's China, vol. 5, p. 8, pi. 



11, fig. 4a. 

 1922. Actinoceras richthofeni Gbabau, Pal. Sinica, ser. B, vol. 1, fasc. 1, p. 75, 



pi. 7, figs. 1, 3 ; pi. 9, figs. 4, 6-8. 

 1927. Actinoceras richthofeni Kobayashi, Jap. Journ. Geol. and Geogr., vol. 5, 



no. 4, p. 191, pi. 20, fig. 8 ; pi. 21, figs. 3 a-b, and 4. 



From an examination of the previously published figures, as well 

 as Plate 20, Figure 10, it will be noted that the inner parts of the 

 septa of this well-known species are in contact with both overlying 

 and underlying connecting rings for a rather considerable width 

 in the area immediately surrounding the passage of the siphuncle 

 through the septa. Moreover, these pictures and my specimen do 

 not show any traces of distinct septal necks, all of which are char- 

 acteristic features of the genus Arinenoceras and are the criteria 

 by which Foerste separated this genus from Actinoceras. There- 

 fore, I have changed the original generic name of this species. 



Description. — The only specimen I have is figured on Plate 20, 

 Figure 10. It is 81 mm long, enlarging at an apical angle of 15° 

 from a lateral diameter of about 23 mm near the base to 40 mm at 

 a point 61 mm farther up. The cross section apparently is elliptical 

 laterally, but only the ventral half of the specimen remains. About 

 12 camerae occupy a length equal to the diameter of the conch. The 

 diameter of the siphuncle is about the same throughout the speci- 

 men; it is 11.5 mm near the top where the lateral diameter of the 

 conch is 40 mm. The siphuncle is subcentral; the distance between 

 the siphuncle and ventral side of the conch is 5 mm near the top. 

 The concavity of the septa averages 9.5 mm. Where the lateral 

 diameter of the siphuncle is 14 mm, that of the passage of the 

 siphuncle through the septum is 9 mm. Between the connecting 

 rings this septum is more nearly horizontal, and its inner margin 



