NATURAL CLASSIFICATION OF THE TRILOBITES 153 



higher order. Placoparia, Areia, and Dindymene, of the 

 Encrinuridse, constitute a group of apparently blind trilo- 

 bites, with narrow marginal free -cheeks, presenting in gen- 

 eral the appearance of Atops, Conocoryphe, Ctenocephalus, 

 etc., of the Conocoryphidse. The somewhat higher genera 

 Cybele and Encrinurus have intermediate or transitional 

 characters leading to the other families. The Cheiruridaa 

 show a greater amount of differentiation and progressive and 

 regressive evolution than any other in this order. Crotalo- 

 cephalus arid Sphcerexochus seem to express the highest de- 

 velopment, and Deiphon and Onycopyge show the effects of 

 over-specialization, resulting in degeneration. The Calym- 

 menidse, in their small eyes and narrow free-cheeks, have 

 decided affinities with the lower genera. The same may be 

 said of Trimerocephalus of the Phacopidse, though the other 

 genera of this family possess large eyes, situated well back 

 and close to the glabella. For these and other reasons, the 

 family is placed at the end of the order, as expressing its 

 highest development. 



Family XI. ENCRINURIDSE Linnarsson. 



Cephalon narrow, transverse. Fixed-cheeks very large. Free- 

 cheeks long, narrow, separate, sometimes with a free plate 

 between the anterior extremities. Sutures extending from in 

 front of the genal angles obliquely forward, and cutting the 

 anterior margin in front of the glabella. Eyes very small or 

 absent. Thorax of from nine to twelve segments, with ridged 

 pleura. Pygidium generally composed of many segments; limb 

 with strong ribs usually less in number than the annulations of 

 the axis. Ordovician and Silurian. 



Including the genera Encrinurus Emmrich (Cromus Barrande), 

 Areia Barrande, Cybele Loven, Dindymene Corda, Placoparia 

 Corda, and Prosopiscus Salter. 



The ConocoryphidaB were shown to be the radical of the 

 order Opisthoparia, and for similar reasons the EncrinuridaB 

 may now be taken as the primitive family of the Proparia. 



