106 BULI^TIN 164, UNITED STATES NATIONAL MUSEUM 



is thickened very conspicuously by the deposition of calcareous 

 matrix, which thins gradually toward the outer surface of the 

 shell. 



C OTThfoHsons. — The general outline of this species appears to 

 relate it to Oi^ioceras orientale^ but the very excentric position of 

 the siphuncle and the smaller septal concavity, as well as the differ- 

 ent manner in which the calcareous matrix is deposited, serve readily 

 in distinguishing it. 



FomuLtion and locality. — Middle Ordovician, Ssuyen formation: 

 In the lower fossil horizon of the black banded limestone, near 

 Chang-tung-kou in the Hsiao-shih district, Liao-tung, Manchuria. 



Holoty'pe.—V.^.'^M. No. 83751. 



ORMOCERAS TAII. new species 



Plate 22, Figuke 8; Plate 40, Figtjbe 10 



Description. — The holotype, 49 mm long, enlarges laterally from 

 a diameter of 17.2 mm at the base to 25.5 mm near the top, the apical 

 angle being estimated at 13°. The dorsal side of the conch has been 

 so weathered away that the real cross section can not be examined, 

 but apparently it is laterally elliptical. The siphuncle is located 

 somewhat excentric laterally. It is estimated that eight camerae oc- 

 cupy a length equal to the lateral diameter of the conch. Where 

 the lateral diameter is 25 mm, the concavity of the septa is 4.5 mm ; 

 the maximum diameter of the siphuncle here is estimated at 5.5 mm, 

 and the height of its camerae averages 3.5 mm. From these di- 

 ameters of the siphuncle and camerae it is estimated that the form 

 of the segments of the siphuncle was nearly globular, being some- 

 what depressed vertically at the contacts between successive seg- 

 ments. The septal necks are very short. The interior of the 

 siphuncle is occupied by calcareous materials, leaving a central en- 

 dosiphuncular passage. The calcareous deposits in the camerae show 

 irregular form; filling densely the basal camerae and becoming 

 gradually less from the base toward the top where only a small quan- 

 tity exists. Surface features of shell unknown. 



The specific name is given in honor of W. Tai, a collector of 

 fossils in Manchuria. 



Comparisons. — At first glance this species appears to be close to 

 Armenoceras tani (Grabau) from the Machiakou limestone, Chihli, 

 and the Ssuyen formation, Manchuria, but further examination 

 shows that it differs in having distinct generic characters — that is, 

 it has a short septal neck, while 0. tail entirely lacks this distinctive 

 feature. 



