156 STUDIES IN EVOLUTION 



prominent glabella is without distinct lobes, and the large 

 pleural extensions of the pygidium do not belong to the ante- 

 rior segment. Its natural place is at the end of the series. 

 F. Cowper Reed 31 has shown (in his memoir on the evolu- 

 tion of Cheirurus and its sub-genera, not including the other 

 genera of the family) that the direct line from Cyrtometopus 

 passes through Cheirurus to Crotalocephalus. The genera 

 Pseudosphcerexochus and Ampliion also have relations with 

 these genera and should be placed here. There is next a 

 group of forms with prominent globular glabellee, leading 

 from Cheirurus to Sphcerocoryphe, and including Actinopeltis, 

 Youngia, and Hemisphcerocoryphe. Staurocephalus should 

 immediately follow these. Sphcerexochus seems to be related 

 to Cheirurus and Actinopeltis. Like them it has two side 

 lobes at the base of the glabella, and the anterior furrows are 

 obsolescent, as in Actinopeltis and Youngia. Lastly come 

 Onycopyge and Deiphon, with their globular glabellse with- 

 out furrows, the spiniform fixed-cheeks, the thoracic and 

 pygidial pleura, and the free-cheeks reduced to almost noth- 

 ing, forming a small part of the doublure of the cephalon. 

 The former genus has four spiniform pygidial pleura, two on 

 each side, but in the latter two are reduced and the remain- 

 ing pair is greatly enlarged. 



Family XIV. PHACOPIDJE Salter. 



Glabella tumid, widest in front. Free-cheeks continuous, 

 united anteriorly. Suture extending from in front of the genal 

 angles inward to the eyes, and then forward around the glabella. 

 Eyes generally large, and always with distinct facets, schizo- 

 cliroal. Thorax with eleven segments, with grooved pleura. 

 Pygidium usually large and of many segments; limb ribbed; 

 margin entire or dentate. Ordovician to Devonian. 



Including the genera and sub-genera PteopsEmmrich, Acaste 

 Goldfuss, Chasmops McCoy, Coronura Hall, Corycephalus Hall 

 and Clarke, Cryphceus Green, Dalmanites Emmrich (Hausman- 

 nia Hall and Clarke), Homalops Bemele and Dames, Monorachos 

 Schmidt, Odontocephalus Conrad, Pterygometpous Schmidt, Sym- 

 phoria Clarke, and Trimerocephalus McCoy. 



