NATURAL CLASSIFICATION OF THE TRILOBITES 139 



anterior margin separately, or rarely uniting in front of the 

 glabella. Compound paired holochroal eyes on free-cheeks, and 

 well developed in all but the most primitive families. 



Including the families Conocoryphidas, Olenidse, Asaphidse, 

 Proetidffi, Bronteidae, Licliadidffi, and Acidaspidee. 



This order is nearly equivalent to group B, or the Asaphini 

 of Salter, which included also the families Calymmenidae and. 

 Harpedidoe, which belong elsewhere. 



The families which are here placed under this order lend 

 themselves quite readily to an arrangement based upon the 

 characters successively appearing in the ontogeny of any of 

 the higher forms. Thus Sao^ Ftyclioparia^ and other genera 

 of the Olenidse have first a protaspis stage only comparable 

 in the structure of the cephalon with the genera of the pre- 

 ceding order, the Hypoparia. Therefore this stage does not 

 enter into consideration in an arrangement of the families 

 of the Opisthoparia. In the later stages, however, there is 

 a direct agreement of structure with the lower genera of 

 this order. The nepionic Sao^ with two thoracic segments 

 (Plate II, figure 2), has a head structure agreeing in essential 

 features with that in Atops or Conocoryphe (Plate II, figures 

 14, 15). A later nepionic stage, with eight thoracic segments 

 (Plate II, figure 3), agrees closely with adult Ptychoparia or 

 Olenus (figures 16, 17). These facts clearly indicate that 

 the family Conocoryphida) should be put at the base of this 

 extensive order. Then, as Ptychoparia and Olenus are more 

 primitive and simple genera than Sao, the}^ as typifying the 

 family Olenida?, should govern its position, which accord- 

 ingly would be next after the Conocoryphida3. In each case 

 a family is considered as represented by its typical and most 

 characteristic forms. It would be impossible to consider the 

 advanced specialized genera of some families as representing 

 their normal facies, for each one lias undergone an indepen- 

 dent evolution, and some characters have reached as great 

 a degree of differentiation as will be found in much hisfher 

 families. 



