38 



ovato- triangular form, the upper side being convex and curving 

 to the ventral margin; the doAum is somewhat flattened, and 

 the lower side sloping with a gentle curve from the lower late- 

 ral angle to the ventral side, which is narrow and sharply 

 rounded. Along the ventral side and a little below the centre 

 there is a narrow, abruptly depressed groove, which extendi? 

 the entire length of the shell. 



Surface of the shell marked by obscure undulations, which 

 are most distinct on the lower lateral angle, also on the lower 

 side by two or three revolving ridges. Fine transverse lines 

 of growth parallel to the margin of the aperture, are visible 

 over the greater part of the surface of the specimen, which is 

 essentially a cast of the interior. 



Geological Formation and Locality, In the Buff limestone 

 of the Trenton limestone group, at Beloit. Wisconsin. 



CEPHALOPODA. 

 GENUS LITUITES, Bbjwn. 



LITUITES UNDATUS, VON OCCIDENTALIS, (n. S.) 



In the "Buff limestone," of Wisconsin, there occurs a 

 large Lituites, which has usually been referred to the 

 Lituites undatas of the Black Biver limestone of -New York. 

 On comparing specimens of the two together, I find several 

 important differences, which could scarcely be expected to 

 happen in so w T ell marked a species as the L. undala, as it oc- 

 curs in the New York rocks. In specimens of about the same 

 size, the volution of the Western one are much wider in pro- 

 portion from the ventral to the dorsal side, they are more flat- 

 tened on the sides and the back is squarely truncated; the 

 New York specimens being rounded on the sides and moder- 

 ately flattened on the back. The volutions in the latter have 

 apparently a greater proportional lateral diameter, and the 

 septa are more distant. 



In consideration of these. differences, I have proposed to in- 

 dicate it as a distinct variety, which hereafter may prove to be 

 specifically distinct. 



Geological Formation and Locality. In the lower part of the 

 "Buff limestone," at Beloit and elsewhere, in Wisconsin. 



LITUITES robertsoni, (n. s.) 



Description. Shell discoid, consisting of three or four volu- 

 tions. Volutions ventricose, very slightly embracing, rounded 

 on the sides and somewhat flattened on the middle of the 



