NATURAL CLASSIFICATION OF THE TRILOBITES 145 



small plate-like pygidium. In ElUptoeephala with eighteen 

 thoracic segments, the cephalon is broader, the third segment 

 is not enlarged except in the young, and the fourteenth to 

 eighteenth segments are narrower and spine-bearing. 



II. Oryctocephalina). — Including Oryctoeephalus, Cteno- 

 pyge, Olenoides^ and Paraholina, with large pygidia and all 

 but the last one or two pleural elements continued into 

 spines; also Eurycare, Angelina^ Peltura, and Protopeltura^ 

 with smaller and shorter pygidia and denticulations of the 

 margins corresponding to the pleural divisions. 



III. OleninsB. — Including Olenus, Agraulus, Liostracus, 

 Aceroeare^ Ptyclioparia, Solenopleura, Ptycliaspis^ Leptoplas- 

 tus, Loganellus, Sphceropthalmus, Parabolinella, Boeekla, Pro- 

 ceratopyge, Ceratopyge, Protypus, PUipsocephalus, Sao, and 

 Triarthrus. All these genera have small or medium-sized 

 pygidia, with from two to eight annulations in the axis. 

 Eyes medium to small, at the ends of distinct eye-lines in 

 all but the latest genera, which preserve this character only 

 during the young stages. Thoracic segments from eleven to 

 eighteen. 



IV. Dikelocephalinfe. — Including Bikelocephalus, Asaphe- 

 lina, and Crepicephalus. Eight or nine thoracic segments. 

 Pj'gidium wide, with the posterior lateral portion often pro- 

 duced into broad spine-like extensions. Dikelocephalus is in 

 many ways related to Ogygia and Asaphus. 



Family VI. Asaphid^ Emmrich. 



Cephalon and pygidium well developed; glabella often ob- 

 scurely limited. Free-cheeks usually separate. Facial sutures 

 extending forward from the posterior edge of the cephalon 

 within the geual angles, and cutting the lateral or anterior 

 margins, occasionally uniting in front of the glabella. Eyes 

 smooth, well developed, sometimes of very large size, even 

 occupying the entire surface of the free-cheeks. Thorax gener- 

 ally composed of eight or ten segments, but varying from fiv^e 

 to ten; capable of enrolment. Fygidiura large, often with 

 wide doublure. Cambrian, Ordovician, and Silurian. 



10 



