FOSSILS OF THE NIAGARA GROUP. 40 1 



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

 at Waukesha and Racine, Wisconsin. I also have seen some large frag- 

 ments of the same species in the collection of Prof. Marcy from Bridge- 

 port, Illinois. 



GENUS TROCHOCERAS, Baerande, Hall. 

 Trochoceras desplainense, M'Chesney. 



PLATE XVI, FIGS. 8, 9, 10. 

 Trochoceras desplainensis, M'Ohesnbt. NevF Palseozoic Fossils, p. 68, Plate viii, fig. 1. 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 differences indicated in the distance of the septa do not seem to be 

 important. This species bears considerable resemblance to Trochoceras 

 trochoides of Barrande, Plate xxix, figs. 16-21. 



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

 Racine, Wisconsin. 



Cab. Nat. 51 



