NATURAL CLASSIFICATION OF THE TRILOBITES 141 



joining and forming part of the anterior margin, as in the 

 glabella of the larval stages of Solenopleura, Liostracus, 

 Ptychoparia, and Sao. Ctenocephalus and Eryx are slightly 

 more advanced, as the glabella no longer marks the edge of 

 the cephalon. In Atops,* Avalonia, Bathynotus, and Carmon 

 the glabella is cylindrical, distinctly defined, and limited 

 within the margin, and in Conocoryphe, Harttia, and Bailiella 

 it narrows anteriorly, and only extends about two-thirds the 

 length of the cephalon. Generally in this family the glabella 

 displays its primitive pentamerous origin. In Bailiella and 

 Oarausia two basal lobes are marked off from the fourth seg- 

 ment by oblique furrows, as in Proetus and Cyphaspis. 



From a phylogenetic standpoint the family Conocoryphidse 

 is at the base of this extensive order. As far as known, all 

 the larval forms in the other families of the Opisthoparia 

 agree in having the narrow marginal free-cheeks, bearing 

 the genal angles. The eye-line is present in most of the 

 adult Olenidse, and in the early stages of all as far as known, 

 so that the general average of the characters in the Conoco- 

 ryphidse represents the main larval features throughout the 

 other families. They show, too, that although primitive in 

 essential structure, differentiation through time has developed 

 secondary features belonging to genera in higher families; as, 

 for example, the basal glabellar lobes in Bailiella. 



Family V. OLENID^ Salter. 



Cephalon larger than the pygidiuin, usually wider than long; 

 genal angles commonly produced into spines. Free-cheeks sepa- 

 rate. Facial sutures extending forward from the posterior mar- 

 gin of the cephalon along the eye-lobes, and either cutting the 

 anterior margin separately or meeting on the median line. Eyes 

 crescentic, reniform, or semi-circular, situated at the ends of eye- 

 lines in all but highest genera. Trunk long, composed of from 



* Atops (type A. truineatus Eramons) seems to be a valid genus, and differs 

 from Conocoryphe (type C. Sulzeri Schlotheim) in its glabellar characters, greater 

 number of thoracic segments, and much smaller pygidium with fewer segments. 



