NATURAL CLASSIFICATION OF THE TRILOBITES 120 



has eight thoracic segments, Plate II, figure 3, the characters 

 of the cephalon accord closely with Ptychoparia and Olenus. 

 showing tliat these genera should precede it in arranging the 

 genera of the family Olenidse. Evidence is thus furnished 

 for the proper position of the first two families of the order. 

 Now, if the relative values of the differentiation of the 

 glabella, the position of the eyes, and the size of the free- 

 cheeks are considered in the light of the preceding analyses 

 of these features, the remaining families of the order, as 

 represented in their typical genera, naturally arrange them- 

 selves as indicated in Plate II, figures 18-23. There result 

 (1) the Conocoryphidse (represented by Atops and Cono- 

 coryphe, figures 14, 15) ; (2) the Olenidse {Ptychoparia and 

 Olenus, figures 16, 17); (3) the Asaphidse (Asaphus and 

 lUcenus, figures 18, 19); (4) the Proetidae (Proetus, figure 

 20); (5) the Bronteidse {Bronteus, figure 21); (6) the Lichad- 

 idse {Lichas, figure 22); and (7) the Acidaspidse (Acidaspi«, 

 figure 23). 



For the Proparia similar results are brought out by the 

 study of the ontogeny of Dalmanites and by comparisons 

 with the characters governing the sequence of families in the 

 Opisthoparia. The narrow marginal free-cheeks place the 

 Encrinuridae and Calymmenidae as primitive. The small or 

 obsolete eyes and the larval form of the glabella in the former 

 further show that this family should be placed at the begin- 

 ning. The nepionic Dalmanites, with seven thoracic segments, 

 has a head structure very similar to the adult Cheirurvs 

 {Eeeoptocheile), figure 28, thus making the Cheiruridse pre- 

 cede the Phacopidee. The arrangement of families under the 

 Proparia accordingly will be (1) the Encrinuridse (Placopari" 

 and Encrinurus, Plate II, figures 24, 25) ; (2) the Calym- 

 menidse {Calymmene and Dipleura, figures 26, 27); (3) the 

 Cheiruridse (Cheiritrus (Eeeoptocheile), figure 28); and (4) 

 the Phacopidse (^Dalmanites, Chasmops, Acaste, Phacops, 

 figures 29-33). 



The sequence of families in the most primitive order, 

 Hypoparia, may now be easily disposed of. The genera are 



