Contributions to Palaeontology. 



171 



The central portion of the posterior part is limited by a 

 faint curving groove ; and anterior to its limit there is a 

 slight elevation, which may have been a node on the sur- 

 face of the crust. The posterior margin, just within the 

 angles, is produced in a minute node. The marginal rim 

 gradually expands from the posterior angles to the front, 

 where it becomes well defined. 



The pygidium is of the same form as the head, slightly 

 truncate at the anterior angles, and the marginal rim nar- 

 rower towards the articulating border : the central part is 

 slightly more elevated, and limited by furrows diverging 

 from the anterior margin. 



In the median line, at a point one-third the length from 

 the anterior margin, there is a distinct elongate node. 



Fig. 23. The head, about three times enlarged. 

 Fig. 24. The pygidium, enlarged in the same degree. 



This species occurs in a yellowish or light drab-colored sandstone, 

 on the shores of Lake Pepin. It is a well-marked species, and 

 readily distinguished from the others by the shallow grooves on both 

 parts, and the distinct node upon the pygidium. 



In the part which I have termed the head, there is some appear- 

 ance of an elongate glabella in front of the curving groove, but the 

 indications are too faint to be satisfactory. 



This species, and the Agnostus disparilis, occur in beds near the 

 middle of the formation ; while A. joseplia occurs in beds which 

 are apparently somewhat lower in the series, but the precise rela- 

 tions have not been determined. 



AGNOSTUS DISPAMLIS ( n. s.). 

 PLATE V. FIGS. 25, 26 & 27. 



Head semielliptical, convex towards the posterior side 

 and abruptly sloping to the front ; length and breadth 

 nearly as three to four ; a little concave on the posterior 

 or articulating margin, and marked near the edge by a 

 narrow groove on each side of the middle, the centre a 



