u 



ONCOCERAS abruptum, (n. s.) 



Description. Shell small, gradually expanding from the 

 outer chamber, and contracted again at the aperture. Very 

 little curved, transversely round-oval, the greatest diameter 

 being in a dorso-ventral direction, the two diameters being as 

 seven and eight. Septa but little concave, not very distant, 

 there being nine iii the length of three-fourths of an inch from 

 the outer chamber, counting on the side. Siphuncle dorsal. 



Surface marked by longitudinal ridges, the remains of which 

 are preserved on the cast. 



This description is drawn from two fragments, one of which 

 is nearly an inch and a half long, retaining eleven of the septa 

 and a portion of the outer chamber; but the abrupt expansion 

 of the shell, together with other characters, are sufficient to 

 distinguish it from any described species. The transverse 

 diameter of one fragment, where broken off, at the smaller end 

 is seven-sixteenths of an inch, and at a distance of three- 

 fourths of an inch it has increased to a diameter of seven- 

 eights of an inch. 



From the 0. constrictum, of the Trenton limestone of New 

 York, it differs in its more closely arranged septa, which are 

 not arched forward on the dorsum as in that species, and also 

 in its greater proportional transverse diameter. 



Geological Formation and Locality. In the Trenton limestone 

 group, at Platteville, and in the same position at Beloit, Wis- 

 consin. 



ONCOCERAS plebeium, (n. s.) 



Description. Shell of medium size, rapidly expanding in the 

 apical half, less rapidly in the middle, and again contracting 

 near the aperture. Transverse section ovate, the diameter as 

 seven to eight and a half, the longest diameter in the dorsV 

 ventral direction, narrowest at the dorsum. Septa at medium 

 distance, there being six in a space equal to their lateral diam- 

 eter, little arched forward on the back, and but moderately 

 concave. Siphuncle dorsal, of medium size, expanded in the 

 chambers. 



Surface unknown. 



This, species is subject to some variation in its curvature at 

 different stages of growth, and also in the transverse diameter, 

 some specimens being more compressed than others. It re- 

 sembles 0. constrictum, of the Trenton limestone of New York, 

 in the unequal expansion, and in the flataess of the septa; but 

 the expansion is not so 'abrupt, and the transverse section i« 

 proportionally much narrower. 



