GENUS TROCHOCERAS. 359 



This is one of the most robust species of the genus known in our rocks, 

 and, were the outer chamber entire, would measure full seven and a half 

 inches in height, with a transverse measurement of more than five and a 

 half inches. The dorso-ventral diameter of the base of the chamber of 

 habitation is two inches and eight-tenths, with a transverse diameter at the 

 same point of two inches and three-tenths. It is likewise remarkable for 

 its great curvature and central siphuncle. 



The flattening of the sides gives the shell in some respects the aspect of 

 Phragmoceras ; but the curvature is so broad and regular, and the outer 

 chamber so extended and gradually expanding, that it presents an aspect 

 more like Cyrtoceras. 



Formation and locality. In limestone of the Niagara age, at Waukesha, 

 Wisconsin. 



GENUS TROCHOCERAS (Barrande, Hall). 



TROCHOCERAS DESPLAINENSE. 

 PLATE 16 (7), FIGS. 8, 9 & 10. 

 Trochoceras de^plainensis : M'Chesney, New Palaeozoic Fossils, page 68. 1860. 



Shell dextral, trochiform, making a little more than two volutions, the 

 apex rising to about the same plane with the top of the adjacent 

 volution, gradually expanding, and the other chamber continued in a 

 more nearly direct line. Section of the volutions essentially circular ; 

 siphuncle small, central. The three outer septa measure on the dorsum 

 seven-eighths of an inch. Surface marked by strong oblique angular 

 ridges which are curved backward on the dorsum, gradually increasing 

 in distance, and finally there is a considerable space below the aperture 

 marked only by lines of growth. The intervals between the annulations 

 are regularly concave. 



I have identified this form with T. desplainense, M'C. ; having before 

 me a cast of the specimen described and figured by that author, in which 

 about two volutions can be distinguished. The original of the cast had 

 apparently been a little flattened from pressure ; and I have a specimen, 

 not figured, presenting a similar aspect. Another specimen, having its 

 natural proportions preserved, is more rotund. It does not appear that 

 there have ever been much more than two volutions in the full grown 

 shell, and the section is essentially circular, though the lateral diameter 

 may be a little greater when the dorso-ventral diameter is measured to the 

 bottom of the depression between the annulations, but not otherwise. The 

 diff'ereuces indicated in the distance of the septa does not seem to be 

 important. 



This species bears considerable resemblance to Trochoceras trochoides of 

 Barrande, Plate 29, figs. 16-21. 



Formation and locality. In limestone of the age of the Niagara group, 

 Racine. Wisconsin. 



