FOSSILS OF THE NIAGARA GROUP. 407 



Cyrtoceras brevioorne, n. s. _ 



PLATE XVIII, FIGS. 8, 9; PLATE XXV, FIG. 14. 



Shell small, gently curving and very rapidly expanding from the apex, 

 •the diameter of the outer chamber nearly equaling one half of the 

 entire length of the shell ; septa somewhat closely arranged, curv- 

 ing upward on the dorsum, moderately concave, the four outer ones 

 .measuring five-eighths of an inch on the back and less than half an 

 inch on the ventral side. Siphuncle small, and lying close to the 

 dorsal margin. Surface of shell unknown. 



This species is the most rapidly expanding form of any yet discovered 

 in these rocks, and may be easily distinguished by this feature alone. 

 The specimen figured retains a single chamber besides the outer one, the 

 remaining portions being restored from the impression in the adhering 

 stone. Another fragment retains the chamber of habitation, and six of 

 the septa below. The fossil is associated with a smaller species of Cyr- 

 toceras {C. pusillum), and a Trochoceras ; and is not an abundant form, 

 so far as known. 



Formation and Locality. — In limestone of the age of the Niagara group, 

 at Racine, Wisconsin. 



Cyrtoceras pusillum, n. s. 



Shell small, slender, making half a volution, somewhat rapidly expand- 

 ing near the aperture ; section broadly elliptical ; the length of the 

 chamber of habitation, as preserved, about equal to once and a half 

 its lesser diameter. Septa moderately concave, arching forward on 

 the back so as to give a depth once and a half as great as on the 

 sides. Siphuncle dorsal. Surface unknown. 



Formation and Locality. — In limestone of the age of the Niagara group, 

 at Racine, Wisconsin. 



Cyrtoceras laterale, n. s. 



PLATE XVIII, FIGS. 4, 5, 6. 



Shell rather above the medium size, very gently curving, and the sides 

 abruptly expanding above the middle, the ventral line being nearly 

 straight, dorsal line more strongly arcuate ; shell more rapidly 

 expanding transversely than in the opposite direction, giving to the 



