NORTH AMERICAN INDEX FOSSILS. 



tt- Pygidium narrow, its two or three spinose segments turned 



abruptly backward XIX. Zacanthoides. 



44. Eyes short cc. 



cc. Raised marginal rim anterior to glabella, bounded inwardly by a 

 furrow. (This concave or flattened margin often bounds the 



entire cephalon and pygidium) f-|"(-. 



ttt- Pleura strongly angular dorsoventrally...XXIV. Strenuella. 



tlf. Pleura not angular ***. 



***. Glabella extending nearly to anterior border of cephalon. 



A"- 



d/^ . Genal spines absent XXVIII. Peltura. 



/^" . Genal spines present d^. 



c'^ . Genal spines set forward on the free cheeks .j. 



j>. Fixed cheeks wider posteriorly than anteriorly. 



XXX. Sjhceropihalmus. 



J. Fixed cheeks about equal in width anteriorly 



and posteriorly c. 



c. Pygidium large, ending in long spines. 



XXIX. Ctenopyge. 



c. Pygidium small, ending in very short spines. 



XXXI. Leptoplastus. 

 c" . Genal spines set at the posterolateral end of the 



free cheeks 4. 



^. Eyes set close to glabella d. 



d. Eyes opposite posterior pert of glabella. 



XXXIII. Dikellocephalus. 



d. Eyes opposite anterior part of glabella i). 



I ) . Marginal fold prominent. 



I XXVII. Parabolina. 

 I XXVIIA. Parabolinella. 

 I). Marginal fold not prominent. 



XXVI. Chariocephalus. 



4. Eyes set far from glabella XXV. Ptvchaspis. 



^^**. Glabella about three fourths the length of the cephalon or 



less 5^- 



5^^. Pygidium with posterolateral spines d^''. 



d/' . Pygidium broad, fan-like. 



XXXI 1 1 . Dikellocephalus. 



. d^\ Pygidium small XXXII, Crepicephalus. 



i/f. Pygidium without spines d' . 



t.". Genal angles with spines XXI. Ptychoparia. 



e^^. Genal angles pointed XXII. Solenopleura, 



e^^. Genal angles rounded XXXVII. Asaphiscus. 



cc. No raised marginal rim anterior to glabella tttt- 



tttt- Pygidium large ****• 



****. Side lobes of pygidium segmented 



XXXIII. Dikellocephalus. 



****. Side lobes of pygidium unsegmented XL. Asaphus. 



•j-j-j-f. Pygidium comparatively small S** 



5*. Glabella reaching nearly to the cephalon. 



XXXIV. Triarthrus. 



