78 NEW YORK STATE MUSEUM 



O. columnare and O. angulatum Hall, and has even been described 

 under four different names by McChesney himself. A comparison with 

 the descriptions and figures given by these authors indicates that that 

 shell has the same rate of tapering as O. trusitum and the same 

 small cylindric ventrocentren siphuncle. The septa show about the same 

 convexity but in all the figured specimens are farther apart than in 

 O. trusitum. 



The surface sculpture of O. angulatum and O. scammoni is 

 described as consisting of angular, equidistant, longitudinal ribs about one 

 line distant when the shell is an inch in diameter. Such ribs are not 

 observable in any of the New York specimens. 



In the collection from Rochester is a single short fragment which 

 has a much slower rate of tapering (6°), the siphuncle is relatively large 

 and only i mm distant from the ventral margin at a diameter of 8 mm, 

 and the septa are very closely arranged, 1.5 mm apart at the same diameter. 

 It is possible that this specimen represents the apical part of O. selwy n i, 

 though there is too little known of this specimen on the one hand and that 

 species on the other, to assert the identity positively. 



Orthoceras trusitum is represented by a large number of 

 fragments (upward of 30) from the Rochester Guelph and is of relatively 

 frequent occurrence in the lower Shelby dolomite. In the upper Shelby 

 bed it is rarer. 



Orthoceras rectum Worthen 



Plate 12, fig. 9 



Orthoceras rectum Worthen, Geology of Illinois. 1875. 6: 504, pi. 26, fig. 3 

 Orthoceras crebescens Hall & Whitfield, Geology of Ohio. 1875. 2:148, 

 pi. 9, fig. 2 



Worthen's original description of this species is the following : 



Shell of medium size, very gradually tapering, septa moderately con- 

 cave, two of the intervals being a little less in width than the diameter of 

 the shell. Length of specimen with 12 septa preserved, 8.87 inches, length 

 of outer chamber about 3 inches. Surface markings and siphuncle 

 unknown. 



