252 NORTH AMERICAN INDEX FOSSILS. 



Literature. 



1847-1888. Hall,J., and Hall, J., and Clarke, J. M. N. Y. Pal., 

 I (Ordovicic), 2 (Siluric), 3 and 7 (Devonic). 



1884-1908. Walcott, C. D. U. S. G. S. Bulls. 10 and 30 (N. Amer. 

 Cambric) ; Monog. 8 (Nevada); U. S. G. S. Ann., 10 (Cambric), 

 Smithsonian Miscellaneous Collections, Vol. LIII., 1908. 



1897. Clarke, J. M. Pal. Minn., Pt. 2 (Ordovicic). 



1901. Van Ingen, G. Sch. Mines Quart., 23 (Batesville, Ark., 

 Siluric). 



1903. Kindle, E. M. Ind. Geol. Surv., 28. 



1905. Raymond, P. E. Annals Carnegie Mus., 3 (Chazy, eastern 

 N. Amer.). 



1907. ^Velle^, Stuart. Palaeontology of the Niagaran Limestone in 

 the Chicago Area. The Trilobita. Bull. Chicago Acad. Sciences, IV., 

 Pt. II. (with complete bibliography of Siluric species of America). 



1882- 1903. Matthew, Geo. F. Fauna of the St. John Group 

 (Proc. Ro)'al Soc. of Canada), 1903, Report on the Cambrian Rocks 

 of Cape Breton and numerous papers in Canadian Roy. Soc. Proc, 

 etc. . 



1890. Vogdes, Anthony W. A Bibliography of Paleozoic Crus- 

 tacea from 1698 to 1889. Bull. U. S. Geol. Survey, No. 63. 



Artificial Key to the Genera. 



A. Free cheeks on under side, usually not showing above. Compound eyes absent ; 



simple ones (mere dots) may occur on each free cheek. Hypoparia I. 



I. Glabella prolonged beyond the anterior edge of the cephalon V. Ampyx. 



I. Glabella not prolonged beyond the anterior edge of the cephalon i. 



1. Small (cephalon or pygidium one fourth inch or less in length) a. 



a. Segmentation of axis of pygidium well marked II. MicrodiscMS. 



a. Axis of pygidium not segmented I. Agnostus. 



i> Moderately large. Cephalon with broad, pitted border b. 



b. Border continued backward and slightly tapering III. Harpes. 



b. Border abruptly contracted at genal angles into true spines. 



IV. Trinucleus. 



B. Free cheeks showing above, always bearing the genal angles. Compound eyes 



(never distinctly facetted ) on the free cheeks, usually present. Opisthoparia..II. 



II. Glabella of cephalon and axis of pygidium distinctly defined 2. 



2. Eyes absent. Glabella distinctly tapering anteriorly c. 



c. Axis wide H. 



II. Axis twice the width of a lateral lobe IX. Bathynotus. 



II. Axis slightly less than the width of a lateral lobe VIII. Atops. 



c. Axis narrow 22. 



22. Lobe present in front of glabella VII. Ctenocephalus. 



22. No lobe present in front of glabella VI. Conocoryphe. 



2. Eyes present ; d. 



d. Glabella single [i. <?., without separated side lobes), prominently enlarg- 



ing anteriorly 33- 



