152 STUDIES IN EVOLUTION 



the basis of priority, Ceratocephala is the first distinctive 

 name ever applied to the group, and is therefore entitled 

 to full generic recognition. He further recognizes Odonto- 

 pleura, Acidaspis, Dicranurus, Selenopeltis, and Ancyropyge in 

 the sub-ordinate position of sub-genera under Ceratocephala. 



Order C. PROPAEIA, nov. ord. 

 (jTpo before, and irapeid cheek piece.) 



Free-cheeks not bearing the genal angles. Facial sutures 

 extending from the lateral margins of the cephalon in front 

 of the genal angles, inward and forward, cutting the anterior 

 margin separately or uniting in front of the glabella. Com- 

 pound paired eyes scarcely developed or sometimes absent in 

 the most primitive family, well-developed and schizochroal in 

 last family. 



Including the families Encrinuridse, Calymmenidae, Cheiru- 

 ridee, and Phacopidse. 



Salter's first division, Phacopini, included the two families 

 Phacopidse and Cheiruridse. The Calymmenidse were placed 

 in his second division, the Asaphini. 



This is the only order of trilobites which apparently begins 

 within the known Paleozoic, and, unlike the other orders, it 

 had no pre-Cambrian existence. The earliest forms of the 

 Proparia came at the close of the Cambrian, in the lower 

 Ordovician. Its greatest generic differentiation Avas attained 

 early. There is a rapid decline in the Silurian and Devo- 

 nian, and only one or two genera extend to the beginning of 

 the Carboniferous. 



In the Opisthoparia it was demonstrated that the Cono- 

 coryphida3 formed the natural base or most primitive family 

 in the order, and was distinguished by the narrow marginal 

 free-cheeks and the absence of well-developed eyes. It is 

 of much interest and importance to be able to recognize, in 

 the Proparia, a similar primitive family having characters in 

 common with the former, but still clearly belonging to the 



