148 STUDIES IN EVOLUTION 



basal lobes defined by oblique furrows in front of the neck seg- 

 ment. Free-cheeks large, separate. Sutures extending from 

 the posterior margin inward to the eyes, and then forward, cut- 

 ting the anterior margins separately'. Eyes prominent, often 

 large. Thorax of from eight to twenty-two free segments, with 

 grooved pleura. Pygidium usually of many segments; pleural 

 and axial portions strongly grooved; margin entire or dentate. 



Ordovician to Permian. 

 Including the genera and sub-genera Proetus Steininger, 

 Arethusina Barrande, Brachymetopus McCoy, Celmus Angelin, 

 Cordania Clarke, Crotalurus Volborth, Cypliaspis Burmeister, 

 Dechenella Kayser, Crrifflthides Portlock, Pha'etonella Novak, 

 Phillipsia Portlock, Prionopeltis Corda, Pseudophillipsia Gem- 

 mellaro, SchmidteUa Tschernyschew, Tropidocoryphe Novak, and 

 Xiphogomium Corda. 



The genera of this family readily fall into a series express- 

 ing more or less closely the development and specialization 

 of various characters. Arethusina is the only genus retaining 

 the archaic eye-lines, and both on this account and for the 

 comparatively forward position of the eyes (itself a nepionic 

 character), together with the large number of thoracic seg- 

 ments, it stands near the base of the series. 



The eyes gradually approach the axis, and move back- 

 ward through the genera Tropidocoryphe, Cyphaspis, Proetus, 

 Prionopeltis, Phillipsia, and Griffithides. Concurrent with this 

 variation, there is a reduction of the fixed-cheeks and exten- 

 sion of the glabella. In Arethusina, Tropidocoryphe, Cor- 

 dania, and Cyphaspis the fixed-cheeks are about the size of 

 the free -cheeks, and occupy a large portion of the cranidium. 

 They are more reduced in Proetus and Prionopeltis, and in 

 Phillipsia and Grriffithides they form only a narrow border 

 to the glabella. The lobation of the glabella varies greatly, 

 and few species retain evidences of its original segmental 

 nature. Some Proetus and Dechenella show this feature, but 

 in many Phillipsia and Griffithides the elements cannot be 

 made out. In Proetus there is often a small accessory lobe 

 developed at the ends of the neck ring, which is only of 



