ORDOVICIAN FAUNAS. 133 
Champlain. The specimens from the Trenton limestone of Minne- 
sota, referred provisionally to this species by Clarke,* do not possess 
the nearly semi-circular head and pygidium of the Fort Cassin and 
New Jersey specimens, and are probably specifically distinct. 
ILLAENURUS COLUMBIANA DN. sp. 
Plate V., Figs. 1-4. 
Description.—Head semi-elliptical in outline; glabella subquad- 
rate, strongly convex, occupying the entire width of the cranidium 
in front of the eye-lobes. The dorsal and occipital furrows nearly 
obsolete. Fixed cheeks small, subtriangular, restricted to the region 
back of the eye-lobes. Eye-lobes prominent, centrally located. Facial 
suture cutting the anterior margin of the head nearly in line with 
the base of the eye-lobes, passing in a nearly straight line to the 
anterior margin of the eye-lobe, and, after encircling this lobe, 
curving gently outward to the posterior margin. Free cheeks about | 
one-third longer than wide, genal angles broadly rounded. On ex- 
foliated specimens an indistinct, flattened rib extends the entire 
length of the head along its median line. Pygidium semi-elliptical in 
outline, axis ill-defined, with three or four nearly obsolete segments. 
Plure smooth, convex, segments obsolete. Surface of test smooth, 
except near the outer border of the free cheeks, where it is finely 
pitted, and near the genal angles, where it is marked with fine, raised 
lines parallel with the border. 
The dimensions of a large cranidium are: length, 23 mm.; width 
at eye-lobes, 22 mm. A large pygidium is 13.5 mm. long and 17 mm. 
wide. 
Remarks.—This is one of the commonest species in the Beekman- 
town fauna near Columbia, and the cranidia occur in all sizes, from - 
3 or 4 mm. in length to that of the large specimen illustrated, having 
a length of 23 mm. The pygidia are not so abundant as the cranidia. 
Four species of the genus I/le@nwrus have previously been described : 
I. quadratus Hall, from the Potsdam sandstone of Wisconsin; J. 
convecus Whitf., from the Lower Magnesian limestone of the same 
State; 7. dia Walcott, from the Upper Cambrian of Texas, and J. 
* Pal. Minn., pt. II., p. TOT. 
