GEOLOGICAL SUKVEY OF THE TEREITOEIES. 481 



placed remote from the glabella, and about their own length from the 

 posterior margins of the cheeks ; iialpebral lobes very small, and lower 

 than the fixed cheeks; movable cheeks sloping abruptly laterally; 

 facial sutures not clearly seen anteriorly, but apparently cutting the 

 margin nearly on a line with each eye ; while behind they are directed 

 at first obliquely outward and backward, after which they curve back- 

 ward so as to cut the posterior margin of the head, just within the inner 

 edge of the little posterior lateral spines. 



Thorax consisting of seven segments; mesial lobe narrow, rather 

 prominent, and gradually tapering; lateral lobes depressed or flattened; 

 pleurse with broad, rounded furrows.* 



Pygidium nearly semicircular, or about three-fifths as long as wide, 

 regularly rounded behind, and rather straight across in front, except- 

 ing laterally, where the anterior margin rounds backward somewhat; 

 quite convex, but not so much so as the cephalic shield. Mesial lobe prom- 

 inent, cylindrical, equaling five-sixths of the entire length of the pygi- 

 dium, with an abrupt posteriortermination, rather decidedly more prom- 

 inent than the lateral lobes, and showing about four obscurely-marked 

 segments. Lateral lobes sloping ofl" laterally and behind, where they 

 are provided with a flattened, somewhat thickened margin, that is armed 

 by six very short, small serrations on each side, directed obliquely 

 backward; each showing four very obscurely-defined, broad, depressed 

 segments that do not extend out upon the flattened and serrated margin. 



Surface of both cephalic shield and pygidium showing, under a mag- 

 nifier, a slight granular appearance, as if covered by minute projecting 

 unequal grains, with smaller pits scattered among them. Obscure traces 

 of strige are also sometimes seen around the margin of the cheeks. 



Judging from the imperfect specimens seen, ii medium-sized entire 

 specimen of this species was probably a little over 1 inch in length, by a 

 breadth of 0.70 inch, and a convexity of near 0,20 inch. 



The cephalic shield and pygidium here described are not positively 

 known to belong to the same species; but judging from the fact that 

 they occur associated together, and agree well in size, convexity, pro- 

 positions, and particularly in the peculiar kind of surface-granulations, 

 there is little room for doubting that they really belong to the same 

 trilobite. This conclusion is also strengthened by the fact that no other 

 cephalic shield and pygidium in tlie collection, not known to belong to 

 other species, correspond with them in these respects. 



The pygidium resembles one figured by Mr. Billings in his Palseozoic 

 Fossils of the Canadian Survey, page 405, Fig. 384, and doubtfully re- 

 garded by him as belonging to a Dikelocej)halus ; but the serrations of 

 the margin in our species are smaller and less prominent, the middle 

 lobe less elongated, more obtuse behind, and has its segments much 

 less distinctly defined ; while the segments of its lateral lobes are also 

 much more obscure. Its anterior lateral angles are also more rounded 

 ofl". From the similarity of the two, however, there can be little doubt 

 that they are allied species of the same genus, in which opinion Mr. 

 Billings fully concurs on examining casts of our species sent to him. 



It is almost beyond doubt that a pygidium figured by Angelin, under 

 the name Corynexochus spinulosus, (see Palseontologia Scandinavien, 

 PI. xxxiii, Fig. 11,) belonging to the same genus as our trilobite ; and, 

 this raises the question whether I ought not to refer our species to that 



*Tlie apecimen from which these characters of the thorax are taken consists of a 

 mold of the interior of the mesial, arid a part of one of the lateral lobes, the glabella, 

 and one of the fixed cheeks, with a part of the pygidium. It does not show the free 

 ends of the pleurae. 

 31 G S 



