172 SIXTEENTH REPORT ON THE CABINET OF NAT. HISTORY. 



Thorax strong; axis elevated, and the extremities of the annula- 

 tions swelling into nodes : ribs strong, grooved in the middle, 

 and bent somewhat angularly backwards in the middle. The 

 anterior facette is abruptly turned inward. 



Associated with the glabellce and cheeks, there are several 

 pygidia Avhich appear to belong to the same species. The specimen 

 fig. 46 is somewhat paraboloid, the axis strong and rounded, and 

 marked by about four very convex rings : the dorsal furrow is 

 deeply impressed by the lateral lobes rising abruptly, and be- 

 coming quite convex in the middle, and thence curving downwards 

 to a narrow flattened border ; marked by three ribs and an ob- 

 scure fourth one, which are grooved from near their origin to the 

 flattened limb. 



This species is common, and even abundant in some beds of greenish 

 gray sandstone at Trempaleau, and near the mouth of the Miniska river. It 

 is chiefiy associated with Ptychasjns ( Dikelocephalus ) granulosa of Owen. 



This is unquestionably identical with the glabella figured by Dr. Owen, 

 Loc. cit., Tab. 1, f. 3 a & 12 ; and Tab. 1 A, f. 5. The pygidium ( Tab. 1, 

 f. 3 h) likewise corresponds with that figured Plate vi, f. 46 ; the former 

 being a somewhat larger individual. 



The glabella of the young of this species bears some resemblance to the 

 glabella of the more convex forms of Dikelocephalus pepinensis ; but the 

 posterior furrow is more oblique, and the second furrow more conspi&uous, 

 while the contour in front is more curved. It likewise resembles, in some of 

 its phases, the D. sjnniger ; but the frontal limb of this is nearly straight, 

 and the glabella is more truncate, while the glabellar furrows very nearly 

 correspond. The form of the fixed cheek, however, distinguishes it at once 

 from either of these, as well as from others of that genus. In young speci- 

 mens, the glabella resembles very closely that of Ptychasjns (Dikelo- 

 cephalus) granulosa of Owen ; but the second glabellar furrow in that 

 species is continued across the glabella, and the posterior lobes are marked 

 by a small tubercle at their extremities, while the front contour is distinc- 

 tive. 



Although referred by Dr. Owen to the Genus Dikelocephalus, this 

 species differs in having the sides of the glabella not parallel throughout, 

 but more particularly in the form of the fixed cheeks, in the movable cheeks, 

 and in the character of the palpebral lobes. 



