354 TWENTIETH REPORT ON THE STATE CABINET. 



the various localities in Wisconsin has not convinced me that we have so 

 large a number of species of a character so similar as those above cited. 



Formation and locality. — In limestone of Niagara age at Racine, Wis- 

 consin, and Bridgeport, Illinois. 



ORTHOCERAS CREBESCENS, (n.s.). 



PLATE 19 (10), FIGS. 1, 2 & 3. 



Shell large, rapidly tapering; transverse section circular; septa deeply 

 concave, four and a half of the intervals being equal to the diameter of 

 the shell. Siphuncle moderately large, central or subcentral, strongly 

 constricted at its junction with the septa, and expanded between. Surface 

 of cast (the usual condition in which the specimens are found), obscurely 

 marked by longitudinal ridges in the most perfectly preserved indivi- 

 duals, their distance varying from a sixteenth to an eighth of an inch ; 

 but these are usually so inconspicuous as not to be observed. 

 In one specimen, preserving a portion of the outer chamber (fig. 1), the 

 septa become much crowded in the the upper part ; but whether this is a 

 constant feature, or only exceptional, cannot be determined. In specimen 

 fig. 2, the septa are oblique to the axis of the shell, caused by the excen- 

 tricity of the siphuncle. 



This species is easily distinguished from the others with which it is 

 associated, by its large size, rapidly increasing diameter, circular section 

 and large siphuncle. The exterior surface characters have not been 

 determined. 



Geological formation and locality. — In limestones of the Niagara group, 

 Racine, Wisconsin. 



ORTHOCERAS ALIENUM (n.s.). 



PLATE 20, A., FIG. 5. 



Shell cylindrical, very gradually tapering, a broad constriction of the 



outer chamber a little below the aperture ; septa deeply concave, about 



four or five in the diameter of the outer one measured : the length of 



the outer chamber equal at least to twice the diameter of the shell. 



Siphuncle central, moderate in size, scarcely constricted at the junction 



of the septa. Surface of the cast smooth ; exterior surface unknown. 



This species is only known in the form of casts of the interior, and is 



distinguished from the others described by its very gradual enlargement 



towards the aperture, below which it is broadly constricted. The shell 



tapers at the rate of a line in an inch of the length. The siphuncle 



is a cylindrical tube, which is scarcely constricted at the junction 



of the septa, and has a diameter of five-sixteenths of an inch where 



the shell is one inch and three-eighths in diameter. 



Formation and locality. — In the limestone of the Niagara group at 

 Hacine, Wisconsin. 



