NATURAL CLASSIFICATION OF THE TRILOBITES 155 



The second group, including Brongniartia, Trimerus, 

 Homalonotus (sens, str.), Plcesiacomia, Dipleura, and Bur- 

 meisteria, agree in having a low, not sharply defined, quad- 

 rate glabella, without distinct furrows or lobes. In general, 

 the axis of the thorax and pygidium is much wider than in 

 the first group, and the pygidium is more elongate and often 

 pointed. 



Family XIII. CHEIRURID^E Salter. 



Glabella well defined. Free-cheeks small, sometimes much 

 reduced. Sutures extending from in front of the genal angles 

 inward to the eyes, and then obliquely forward, cutting the 

 anterior margin in front and each side of the glabella. Eyes 

 usually small. Thorax composed of from nine to eighteen seg- 

 ments, generally eleven; pleura often extended into hollow 

 spines. Pygidium small, with from three to five segments; 

 pleural elements commonly produced into spines. 



Principally Ordovician and Silurian, but extending into the 

 Devonian. 



Including the genera and sub-genera Cheirurus Beyrich, Actin- 

 opeltis Corda, Amphion Pander, Anacheirurus Heed, Ceraurus 

 Green, Crotalocephalus Salter, Cyrtometopus Angel in, Deiphon 

 Barrande, Diaphanometopus Schmidt, Eccoptocheile Corda, Hem- 

 isphcerocoryphe Reed, Nieszkowskia Schmidt, Onycopyge Wood- 

 ward, Pseudosphcerexochus Schmidt, Sphcerexochus Beyrich, 

 Sphcerocoryphe Angelin, Staurocephalus Barrande, and Youngia 

 Lindstrom. 



As in other families, the most primitive genera are those 

 in which the regular pentamerous lobation of the glabella is 

 retained, with the eyes well forward, the free-cheeks narrow, 

 and the fixed-cheeks ample. Diaphanometopus, Anacheiru- 

 rus, Eccoptocheile, and Cyrtometopus agree in these respects, 

 and therefore belong at the beginning of a phylogenetic list. 

 Ceraurus and Nieszkowskia appear to branch off here, being 

 characterized by the narrow transverse form of the cephalon 

 and the great development of the two anterior pygidial pleura 

 into hollow spines directed outward and backward. These 

 features are simulated in Deiphon, in which, however, the 



