74 BULLETIN" 164, UNITED STATES NATIONAL MUSEUM 



is a Plectoceras, since it has the characteristic coiling of the trans- 

 verse riblike folds on the surface of the shell and the large umbilical 

 perforation, and agrees in other details. The Manchurian specimen 

 may be described as follows: 



Description. — ^Volutions of conch apparently in contact with one 

 another, consisting of about four and one-half volutions, of which 

 the living chamber probably occupied a length of about one-third 

 of a volution, though the actual length of the part here preserved 

 is only 125 mm, and the maximum diameter of the conch across its 

 umbilical part is about 240 mm. 



At the larger end of the specimen its dorsoventral diameter is 

 estimated as 46 mm. One volution back from this larger end, the 

 dorsoventral diameter is about the same as that of the former one. 

 Two volutions back from larger end, the dorsoventral diameter 

 diminishes to about 25 mm. The earlier volutions of the conch prob- 

 ably are in contact with one another, and the dorsal side may be 

 finely impressed, but toward the aperture the living chamber may not 

 be in actual contact with the preceding volution, though closely con- 

 tiguous. The maximum prominence of the lateral sides is much 

 nearer to the dorsal side. The dorsolateral angles are very promi- 

 nent, and are estimated at about 70°, but the ventrolateral angles 

 may be broadly rounded. 



The surface of the shell is crossed obliquely by very remarkable 

 transverse riblike folds, which curve backward along the lateral 

 sides of the conch. About four of the folds occupy a length equal 

 to the dorsoventral diameter of the shell. In addition to these 

 oblique riblike folds the surface of the conch is crossed by fine, 

 minute, transverse striae, parallel to the folds. 



This species is characterized by its remarkable dorsolateral angle, 

 very sharp and well-defined riblike folds, and about the same dorso- 

 ventral diameters of the living chamber and the preceding volution 

 as compared with other species of Plectoceras. 



Formation and locality — Lower Ordovician, Wuting formation: 

 In loose limestone bowlder, apparently from the Wuting formation 

 near San-tao-kang-tzu, 1.5 miles north of the Pen-hsi-hu colliery, 

 Liao-tung, Manchuria. 



Eolotype.—U.S.'^M. No. 83696. 



Genus SACTOCERAS Hyatt, 1884 



SACTOCERAS KOBATASHII, new species 



Plate 14, Figube 6 ; Plate 40, Figuee 8 



Description. — The holotype is 54 mm long, enlarging laterally 

 from a diameter of 21.5 mm at its base to 28 mm at a point 44 mm 



