GENUS TROCHONEMA. 000 



The surface, as preserved in a mould of the exterior in the stone, is 

 marked on the upper side of the volution by ten or twelve moderately 

 strong- revolving ridges, which are smaller and more closely arranged 

 toward the suture (where there is a slightly depressed or flattened space). 

 These are crossed by numerous less strong, closely arranged transverse 

 stria), which bend backwards from the suture, and have a strong retral 

 curve on the narrow concave band of periphery. 

 Surface characters of under side undetermined. 



This species is so entirely distinct in its form and surface characters 

 from any other species yet known in our Silurian rocks, that it can be 

 readily distinguished. It is more nearly allied with forms such as P. lucina 

 of the Upper Helderberg and Hamilt(jn groups of New-York. 



Formatian and locality. In limestone of the age of the Nigara group 

 of JSew-York, at Racine, Wisconsin and Bridgeport, Illinois, 



PLEUROTOMARIA IDIA (n.s.). 



PLATE 15 (6), FIGS. 15 & 16. 



Pleurotomaria idia ; Hall, Geol. Survey of Wisconsin, Report of Progress, p. 35, 1861 . 



Shell depressed orbicular, moderately convex above, and broadly umbi- 

 licate beneath, with four volutions which are moderately convex above, 

 and gradually increase in size from the apex, the outer half of the last 

 one being more ventricose and regularly rounded on the periphery, and 

 curving into the broad umbilicus ; aperture, or section of volution near 

 it, broadly ovate. 



Surface characters unknown. 



This species differs from the last in being" less elevated, in its more 



gradually increasing volutions, broader umbilicus and absence of angularity 



on the periphery. 



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



at Racine, Wisconsin. 



GENUS TROCHONEMA (Salter). 



TROCHONEMA PAUPER. 



PLATE 15 (6), FIGS. 5, 6 & 9. 



'*-*-^ """^ Pleurotomaria pauper , page -8#4 (39) . 



The matrix of a specimen of this species, subsequently discovered in the 

 collection, gives a cast of the exterior form and markings of the shell as 

 represented in figures 5 and 6 of pi. 15. The following description will 

 more nearly express the characters of the species. 



Shell small, obtusely conical, the apicial angle from 70 to 80 degrees. 

 Volutions three, four or more, rapidly increasing in size from the apex, 

 rounded above and on the sides, a little flattened on the top below the 

 suture line, the last one subangular below ; suture distinct, not channelled. 



