734 THE PALEONTOLOGY OF MINNESOTA. 



[Ceraurus pleuroxaathentus, 



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^E. 

 Genus CERAURUS, Green, 1832. 

 CERAURUS PLEUREXANTHEMUS (Jreen, 1832. 



Ceraurus pleurexanthemus GREEN, 1832. Monogr. Tril. North Anier., p. 83; cast 33, plate 3, B&f. 10. 

 Ceraurus pleurexanthemus HALL, 1847. Paleontology of New York, vol. i, p. 242, pi. 65, flgs. la-n; 



pi. 66, flgs. 1, 1ft. 

 Ceraurus pleurexanthemus WALCOTT, 1881. Bull. Mus. Comp. Zool., Harvard Coll., vol. viri, p. 211 



pi. 5, flgs. 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, Jauesville and Mineral Point, Wisconsin; Black River 

 horizon, Minneapolis; Trenton horizon, St. Paul; Galena horizon, Kenyon; Hudson 

 River horizon, Spring Valley, Minnesota. 



Subgenus PSEUDOSPH^EREXOCHUS, ScHmidt, 1881. 



1 'SEUDOSPH.SREXOCHU8 TRENTONENSI8, U. 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, Pseudosphcerexochus, 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 3 mm. in width and decidedly flat on the 

 axis. The width of the glabella at its base is 21 mm. The three pairs of glabellar 



Adv. Sin'.. Is Twi nli.-lli IN-i't. N :il>. N:it. Hist.. |>. IB, 1M1: 'I' w I'M I V -foil i 'I li '!').. }>. >%!, pi. viii, fijrs. lf>, ID. IHV.i; 



iloli.Uy f Olilii, vol. II, p. 108, pi. Iv. fl;;s in. 17. 1S7.1. 



