402 REPORT ON THE STATE CABINET. 



Trochoceras costatum, Hall. 



PLATE XXV, FIG. 1.5. 



Trochoceras costatum, Hall. Report Progress Geolog. Survey of Wisconsm for 1860, 1801. 



Shell depressed trochiform, 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 sub- 

 circular ; umbilicus broad and shallow. Siphuncle undetermined. 

 Surface marked by strong sharply elevated annulations, which 

 increase in distance and regularity with the growth of the shell, 

 gradually diminishing 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 the 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 annulations as in the species cited. The sinistral direction 

 of the volutions is, however, a conspicuously distinguishing feature.t 



The species was originally described from imperfect material, and the 

 figure given is from a gutta-percha 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. pulchrum of Barrande, Plate 

 xvii, 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. 



* Originally stated as three or four by mistake. 



t Since it sometimes happens that the Trochoceras desplainense is quite flat upon the upper side, 

 or that the inner volutions are a little depressed below the outer one, it might perhaps be suspected 

 that the T. costatum, of which the umbilical side is represented in the figure, is an exaggerated 

 condition of the former species, with the inner volution still more depressed. This, however, is 

 not true, for the volutions are clearly sinistral, and as represented are in their natural relations. 

 It is, perhaps, the first American species of this character that has been recognized, but M. Bak- 

 RAXDE distinctly describes the two forms, and has figured several species with sinistral spires. 

 In his generic description, he says, " the shell consists of several turns of the spire or of a single 

 volution more or less complete, but exhibiting almost always a marked defect of symmetry. The 

 enrolment is sometimes dextral and sometimes sinistral according to the species, but the dextral 

 forms greatly predominate. In T. asperum we find both modes, varying in individuals, and the 

 same in T. sandbergeri." 



