78 BULLETIN 1G4, UNrTED STATES NATIONAL MUSEUM 



angle of about 14°. The cross section of the conch apparently was 

 elliptical laterally, that of the siphuncle circular. The siphuncle is 

 located excentric laterally. Seven camerae occupy a length equal to 

 the diameter of the conch. Where the diameter of the specimen is 

 20 mm, the segment of the siphuncle has a maximum diameter of 3.6 

 mm at mid height, constricting to 2.5 mm at its passage through the 

 septum. The resultant form of segments of siphuncle is almost 

 globular. Septa are moderately concave. The interior's of the 

 camerae and siphuncle are irregularly filled with calcareous matrix. 

 Formation and locality. — Middle Ordovician, Ssuyen formation: 

 In the lower fossil horizon of the gray banded limestones, near 

 Pen-hsi-hu, Liao-tung, Manchuria. 



Genus ACTINOCERAS Bronn, 1837 



ACTINOCERAS CONCAVUM, new species 



Plate 16, FiGxmE 10 



Description. — A rather good specimen is 80 mm long, enlarging 

 from a diameter of 26.5 mm near the base to 34 mm at the top, the 

 apical angle being 6°. The cross sections of the conch and the siphun- 

 cle were apparently circular, but only the dorsal side of the shell 

 remains. The number of camerae in a length equal to the diameter 

 of the conch is about 6.7. Where the lateral diameter of the conch is 

 34 mm, the concavity of the septa is 10 mm, the length of the camerae 

 is 5.2 mm, and the maximum diameter of the segments of the siphun- 

 cle is 13.2 mm. The septal necks descend slightly below the gen- 

 eral concave curvature of the septa, and their length is usually 0.7 mm 

 or less. The inner margin of the septa is in contact with the lower 

 surface of the preceding connecting rings for a radial width of only 

 2 mm. The interior of the siphuncle is filled with typical actino- 

 ceroid materials, leaving a medium-sized endosiphuncular passage 

 and its narrow diverticula, which bend forward somewhat. The 

 septa are thickened considerably by the deposition of calcareous 

 matrix, which occurs both on the upi^er and under sides of the sep- 

 tum, leaving very narrow, diagonal interspaces in the middle part 

 of each camera. 



Coiwpari&ons. — This species may be compared to Armenoceras 

 richthofeni (Freeh), but differs from it in having greater concavity 

 of the septa and fewer camerae in a length equal to the diameter of 

 the conch, and comparatively longer septal necks. 



Formation and locality. — Middle Ordovician, Ssuyen formation: 

 A black limestone bowlder believed to have come from a part of the 

 Ssuyen formation ; collected in river drift at the Pen-hsi-hu colliery, 

 Liao-tung, Manchuria. 



Eolotype.—ll.^.'^.M. No. 83705. 



