528 bulletin: museum of comparative zoology. 



I. Eye usually more than half the length of the cephalon 

 from the posterior margin. 

 A small form, glabellar expansion I in 4. 



hudsoni Raymond 

 A medium sized form, glabellar expansion I in 7. 



pleurexanthemus Green 

 II. Eye usually about one half of the length of the cephalon 



from the posterior margin millcranus M. and G. 



III. Eye less than one half the length of the cephalon from 



the posterior margin dentatus, sp. nov. 



B. Glabella with parallel sides granulosus, sp. nov. 



Ceraurus pleurexanthemus Green. 



Plate 1, fig. 1; Plate 2, fig. 1, 2, 7. 



Ceraurus 'pleurexanthemus Green, Monthly Amer. journ. geol., 

 1832, p. 560, pi., fig. 10; Monogr. N. A. trilobites, p. 84, pi. 1, fig. 10, 

 cast 33. Hawle & Corda, Prodr. monogr. Bohem. trilobiten, 1847, 

 p. 161, pi. 7, fig. 82. Hall, Pal. N. Y., 1847, 1, p. 242, pi. 65, figs, la-c, 

 le-g (not Id, lh, li, lm, nor pi. 66, figs. la-g). Emmons, Amer. geol., 

 1855, 1, p. 217, pi. 15, fig. lb-e, lg (not la, lh, li, Ik). Walcott, Ann. 

 Lye. nat. hist. N. Y., 1875, 11, p. 155, pi. 11; Bull. M. C. Z., 1881, 

 8, p. 211, pi. 5, fig. 1-6. Clarke, Pal. Minn., 1897, 3, p. 734. Weller, 

 Pal. N. J., 1897, 3, p. 204, pi. 15, fig. 28. 



As may be seen from the citations under this species and the next, 

 the literature is full of references to Ceraurus pleurexanthemus, but 

 original figures are scarce, most authors seeming to find Hall's imper- 

 fect illustrations satisfactory. Barrande, as early as 1852, was led 

 by Hall's figures to believe that two species had been described under 

 one name, 1 but up to the present no one has given the subject any 

 especial attention. There have been two reasons for this neglect of 

 the species. One is, that although it is common at various horizons in 

 the Ordovician, good specimens are exceedingly rare; in the second 

 place, the figures given by Hall are so poor that they do not afford 

 means for fine discrimination, and encourage a quick and loose de- 

 termination of the specimens. 



The Walcott and Dyer collections in the Museum of Comparative 

 Zoology afford abundant material for the study of the various forms 



1 System Silurian dela Boheme, 1852, 1, p. 756. 



