GUELPH FAUNA IN THE STATE OF NEW YORK •jj 



CEPHALOPODA 



obthocebas Breyn 1732 



Orthoceras trusitum sp. nov. 



Plate 10, fig. 25, 26 ; plate 13, fig. 1-10 



Shell straight, thick, regular, slender, tapering at an angle of io° ; of 

 moderate size, diameter of largest fragment 38 mm, transverse section cir- 

 cular or subelliptic. Septa closely arranged, regularly and moderately con- 

 cave, their depth approximately equaling that of the camerae ; about 6 mm 

 distant at the largest diameter, 3 mm distant when the shell has a diameter of 

 25 mm, 2.2 mm when the latter is 15 mm and about 1 mm at the apical end. 

 The sutures are not straight, have a broad saddle on the ventral side but 

 on the opposite side are nearly transverse. Siphuncle very small (3 mm 

 where the diameter of the chamber is 26 mm), tubular and ventrocentren. 

 The living chamber shows the same degree of tapering as the septate por- 

 tion of the shell. The aperture has not been observed. The surface is 

 usually entirely smooth, but when well preserved shows a very faint and 

 fine longitudinal striation. In the largest specimen observed these lines 

 are 1.5 mm apart and consist of broad flat ridges with narrow furrows 

 between. 



One of these specimens was labeled by Mr Arey, Orthoceras sel- 

 wyni Billings. This is probably the specimen so listed in the provisional 

 enumeration of fossils given by the discoverer. While it is true that the 

 specimens under consideration have the same apical angle and depth of 

 camerae as Billings's measurements of that species indicate, the siphuncle of 

 O. selwyni is moniliform, with discoid inflations between the septa, and 

 lies at a short distance from the ventral margin. According to Whiteaves 

 the original specimen of this species is a very imperfect cast and the species 

 therefore incompletely characterized. 



There is some justification for comparing these specimens to O. s c am- 

 mo n i McChesney. The somewhat complicated synonymy of this form 

 given by Whiteaves shows that it has passed under various names, as 



