CONTRIBUTIONS TO PALiEONTOLOGY. 177 



The middle lobe of the thorax is wide and moderately convex ; 

 the lateral lobes narrow, little convex, and the extremities 

 slightly bent backwards, and posteriorly acute. A narrow 

 shallow furrow runs through the middle of the length of the 

 segment. 



Pygidium short, convex in front and marked near its margin by 

 a narrow groove, rounded behind. 



The specimens observed are the casts of dismembered portions of the 

 body ; and occurring in sandstone, it is difficult to determine all the cha- 

 racters. The glabellse, pygidia and cheeks are preserved in considerable 

 numbers in a fragment of sandstone from near Osceola mills on the St. Croix 

 river, from a position near the middle of the formation. 



In more extensive collections made at other localities, no fragment of 

 this species has been observed ; and it is, therefore, with probability, in- 

 ferred that its horizontal range is restricted. 



GENUS TRIARTHRUS? 



SUBGENUS THIAETHRELLA ( ii. g.). 

 TRIARTHRELLA AURORALIS { n. s.). 



PLATE IX. FIG. 13. 



A small and obscure species occurs among the Dikelocephali 

 at Lagrange mountain, which has an elongate semioval glabella, 

 with the fixed cheeks wide and spreading in the posterior limb, 

 and very narrow in front. The glabella shows an obscure indenta- 

 tion at its margin (not so strongly as represented in the figure), 

 and the general expression is like Triarthrus. 



The species can hardly be characterized from the imperfect specimens 

 known, some four or five of which have been observed. 



Fia. 13. The glabella and fixed cheeks. The figure is twice the natural size of the 

 specimen. 



^*^ For notice of a species of Conocephalites belonging to the first 

 group of species indicated in this paper, see Explanations of Plate vii. 



[Senate, No. 135.] 23 



