FOSSILS OF THE NIAGARA GROUP. 403 



Trochoceras notum, n. s. 



PLATE XVI, FIGS. 1, 2. 



The entire shell unknown ; a fragment of the outer volution shows it to 

 have been strongly annulated, with the annulations bending back- 

 ward on the dorsal side. The transverse diameter is greater than 

 the dorso-ventral diameter, and the direction of the volutions has 

 been dextral. The siphuncle is subcentral and small. 



It is distinguished from T. costatum by the dorso-ventral compression 

 of the volutions, and by the dextral direction of the spire. The volu- 

 tions have been in contact, and their number, though unknown, is proba- 

 bly about two. 



It differs from T. desplainense in the dorso-ventral compression of the 

 volutions, and less conspicuous and more closely arranged annulations. 



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

 at Bridgeport, Illinois. 



Trochoceras (Gyroceras) bannisteri, Winch, and Mar. 



PLATE XXV, FIG. 17. 

 Gyroceras bannisteri, Wijichell and Marct; in Mem. Bost. Soc. of Nat. Hist., I, p. 102. 



Shell consisting of about one and a half volutions, which increase in size 

 somewhat rapidly ; spire gently ascending ; umbilicus broad and 

 comparatively deep, the lateral diameter of the volutions being 

 greater than the dorso-ventral diameter. Surface marked by low 

 subangular annulations, which, turning gently backwards on the 

 sides of the shell, are more abruptly bent into a shallow sinus on 

 the dorsum. These annulations apparently become obsolete towards 

 the aperture ; on the other parts of the shell, there are about six or 

 seven in a space equal to the dorso-ventral diameter. 



This species difiers from either of the species described, in its more 

 rapid enlargement from the apex, and in the finer annulations. 



These observations are made upon a cast in gutta-percha, sent to me 

 by Prof WiNCHELL, under the name of Gyroceras bannisteri. The cast 

 has the appearance of a very pretty species of Trochoceras, of more 

 delicate proportions than those described in this paper. 



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

 at Bridgeport, Illinois. 



