170 



Contributions to Palaeontology. 



Glabella prominent, narrow, extending about two-thirds 

 the length of the head, and crossed by a shallow furrow 

 near its anterior end : the posterior lobe is marked by 

 an oblique furrow on each side, and a small node on 

 the summit at the anterior termination. The triangular 

 space on each side, between the tranverse and oblique 

 furrows, is likewise elevated into a low node. The 

 posterior central portion is gibbous, narrowed at the 

 base, with a small tubercle on each side. A narrow 

 longitudinal furrow extends from the apex of the gla- 

 bella to the marginal limb. 



Pygidium of the same form as the head, or a little wider; 

 axis prominent, subquadrangular, wider than long, 

 nearly one-third the length of the pygidium, bearing a 

 node or short spine on its posterior extremity ; sides and 

 body of the pygidium (outside of the axis) highly con- 

 vex. 



In some of the specimens, the sides are a little more 

 straight and parallel than those given in the figures ; and 

 in several specimens, a truncation or slight emargination 

 has been observed in the anterior border. 



Fig. 54. The head of this species, three times enlarged. 



Fig. 55. The pygidium of the same, enlarged in the same proportions as 

 the preceding. 



This species is common in some layers of gray sandstone, asso- 

 ciated with Conocephalites ivisconsensis and Ptychaspis granulosa, at 

 Trempaleau. It occurs also at the mouth of Black river, and else- 

 where on the Mississippi about Lake Pepin. 



AGNOSTUS PARTUS ( n. s.). 

 PLATE V. FIGS. 23 & 24. 



The two extremities of similar form. The head semi- 

 elliptical ; length and width about equal, very convex 

 in the posterior part, and curving downwards to the 

 anterior margin. 



