734 THE PALEONTOLOGY OF MINNESOTA. 
(Ceraurus pleurexanthemus. 
departure toward the true Phacops, leading thence through the little Upper Silurian 
P. trisulcatus Hall, and P. orestes Billings, into the typical forms of the Devonian. 
Chasmops is a genus abundantly represented in the Scandinavian and Russian 
Lower Silurian, but with a single American representative, D. breviceps Hall,* from 
the Hudson River group of Ohio. In this genus the first glabellar lobes are extrava- 
gantly developed at the expense of the other pairs and extend frequently from the 
first glabellar furrows to the occipital ring. 
Family CERAURID A. 
Genus CERAURUS, Green, 1832. 
CERAURUS PLEUREXANTHEMUS (Geen, 1832. 
Ceraurus pleurexanthemus GREEN, 1832. Monogr. Tril. North Amer., p. 83; cast 33, plate 3, fig. 10. 
Ceraurus pleurexanthemus HALL, 1847. Palwontology of New York, vol. i, p. 242, pl. 65, figs. la-n; 
pl. 66, figs. 1, 1h. 
Ceraurus pleurexanthemus WALCOTT, 1881. Bull. Mus. Comp. Zool., Harvard Coll., vol. viii, p. 211_ 
: pl. 5, figs. 1-6. 
This species abounds at several localities, and specimens do not materially differ 
in size and habit from those of the Trenton limestone of New York. It has a very 
considerable vertical range, having been found at the following localities. Lower 
Trenton, or Birdseye limestone, Janesville and Mineral Point, Wisconsin; Black River 
horizon, Minneapolis; Trenton horizon, St. Paul; Galena horizon, Kenyon; Hudson 
River horizon, Spring Valley, Minnesota. 
Subgenus PSEUDOSPHAREXOCHUS, Schmidt, 1881. 
PsEUDOSPHHREXOCHUS TRENTONENSIS, ”. Sp. 
The remains of this fossil are quite imperfect, but the occurrence of this type of 
structure in the Trenton fauna of New York is worthy of notice. 
This division of the Ceraurus group, Pseudosphwrexochus, was introduced for such 
species as differ from Ceraurus in the subtrigonal rather than subquadrate outline 
of the glabella, its convex or bullate contour, the posteriorly directed glabellar 
furrows (the third pair being the largest, but not reaching the occipital furrow), and 
the very large size of the third pair of glabellar lobes; all of which are features in 
strong contrast to their disposition in the restricted genus Ceraurus. . 
Our specimen is a single glabella of large size, regularly convex surface, and 
rather abrupt slopes to the frontal and lateral margins. Its length to the occipital 
furrow is 19 mm., and the occipital ring is 8 mm, in width and decidedly flat on the 
axis. ‘The width of the glabella at its base is 21mm. The three pairs of glabellar 
* Adv. Sheets, Twentieth Rept. N. Y. State Cab. Nat, Hist., p. 16, 1866; Twenty-fourth do., p. 223, pl. viii, figs, 15, 16, 1872; 
Palzontology of Ohio, vol. li, p. 108, pl. iv, figs 16, 17, 1875. 
