CRUSTACEA— TRILOBIT A. 253 



33. Pygidium as long as cephalon or thorax aa. 



aa. Cephalon and pygidium with a flattened margin. 



XLIX. Griffithides. 

 aa. Cephalon and pygidium without a flattened margin. 



L. Bronteus. 



ZZ' Pygidium very short, plate-like XV. Paradoxides. 



d. Glabella single, not prominently enlarging anteriorly 44. 



44, Eyes long and narrow, extending from the margin of the anterior 

 glabellar lobe (or the end of the ocular ridge) nearly to the poste- 

 rior end or obliquely outward bb. 



bb. Sides of glabella somewhat irregular or narrowing anteriorly... f. 



f. Glabella wider than long XVI. Kemopleurides. 



|. Glabella longer than wide *. 



*. Cephalon without genal spine XIII. Ellipsocephalus. 



■5^. Cephalon with genal spine \^'. 



\'^. Cephalon with strong, very narrow marginal rim. 



J XIV. Frotolenus. 

 \ XIVA. Bergeronia. 



V\ Marginal rim absent or not very narrow 2/^. 



7)/\ Pygidium a long spine X. Olenellus. 



2/^. Pygidium a small, unsegmented plate 7. 



I. Anterior three fourths of thoracic segments ab- 

 ruptly longer than posterior one fourth ; without 

 secondary cephalic spines XII. Mesonaces. 



1. Anterior segments not differing from posterior ; 

 with secondary cephalic spines. XI. Holniia. 



2/', Pygidium comparatively broad 2. 



2. Pygidium with posterolateral spines a. 



a. Third or fourth pleural spine much extended. 



XX. Albcrtella. 



a. Pleural spines of equal length. 



XXXII. Crepiaphalus. 

 2. Pygidium without spines b. 



b. Glabella about three fourths the length of the 

 cephalon or less XXI. Ptychoparia. 



b. Glabella extending almost to anterior edge of 



cephalon XXXV. Bathyuriscus. 



2/^. Pygidium unknown. Pleura strongly angular 



dorsoventrally XXI V . Strenuella. 



bb. Sides of glabella straight or subparallel ff. 



ft- Pygidium very broad **. 



**. All annulations of pygidium terminating in short spines.. 2'''. 

 2^' . Outline of cephalon semicircular ...XVIII. Olenoides. 



2'\ Outline of cephalon subtriangular XVII. Neolenus. 



**. Margin of pygidium entire "}/'• 



t/'. Cephalon and pygidium subequal, large. 



XXXVIII. Ogygopsis. 



2/' . Cephalon and pygidium unequal b^-'. 



\i^'. Glabellar furrows present XXI. Ptychoparia. 



h^^. Glabellar furrows obsolescent. 



XXXVI. Baihyurus. 



