360 TWENTIETH REPORT ON THE STATE CABINET. 



TROCHOCERAS COSTATUM. 

 PLATE-a^^FIG.l. 7^ 



Trochoceras costatum : Hall, Geol. Report of "Wisconsin, published Feb. 1861. 



Shell depressed trocliiform, sinistral, greatest diameter about two and 

 a quarter inches ; spire depressed convex ; volutions about one and a 

 half to two*, gradually expanding from the apex ; section subcircular ; 

 umbilicus broad and shallow. Siphuncle undetermined. 

 Surface marked by strong sharply elevated annulations, which increase 

 in distance very regularly with the growth of the shell, gradually dimi- 

 nishing on the last volution and becoming more or less obsolete, or 

 appearing as gentle undefined elevations towards the aperture. Near 

 the apex of the shell there are about twelve of these annulations in th 

 space of half an inch, and on the outer volution opposite the same point 

 there are barely six in the same space. 



This species is a little less in size than the T. desplainense ; the annula- 

 tions are more numerous and more sharply elevated, not increasing in size 

 on the outer volution beyond the point opposite the apex of the shell. 

 Taking a single volution from near the apex, there are nearly twice as 

 many as in the species cited. The sinistral direction of the volutions is, 

 however, a conspicuously distinguishing feature.! 



The species was originally described from imperfect material, and the 

 figure given is from a guttapercha cast in a well preserved impression of 

 the exterior of the lower or umbilical side of the shell. 



This species may be compared with T. yulchrum of Barrande, plate 

 17, figs. 8-16, but is more finely costate. 



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

 at Racine and near Milwaukee, Wisconsin. 



TROCHOCERAS NOTUM (n.s.). 

 PLATE 16 (7), FIGS. 1& 2. 



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

 have been strongly annulated, with the annulations bending backward 

 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 sub-central and small. 

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



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



have been in contact, and their number, though unknown, is probably 



about two. 



It diff'ers from T. displai?iense 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. 



* Originally stated as three or four by mistake. ] See note on next page. 



