626 



AKTHROPODA 



SUB-KINGDOM VII 



cephalon subtriangular, without pitted limb; glabella large, prominent, narrow be- 

 hind and enlarged anteriorly, often produced into a frontal spine. Genal angles 

 spiniform ; thoracic segments five or six. Chiefly Ordovician, rare in Silurian ; 

 -Europe and North America. 



Order 2. OPISTHOPARIA. Beecher. 



Free cheeks generally' separate, always bearing the genal angles. Facial sutures 

 extending forward from the posterior part of the cephalon within the genal angles, and 

 cutting the anterior margin separately, or more rarely uniting in front of the glabella. 

 Compound paired holochroal eyes on free cheeks, and well developed in all but the most 

 primitive family. 



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, Ptychoparia, and other genera of the Olenidae 

 have first a protaspis stage only comparable in the structure of the cephalon with the 

 genera of the preceding order. Therefore this stage does not enter into consideration 



FIG. 1288. 



Cephala of Opisthoparia. A, Atops. B, Conocoryphe. C, Pti/<-liop<n-ii.i. 

 G, Proetus. H, Bronteus. I, Lichas. J, Addaspis (after Beecher). " 



D, Olenus. E, Asuplms. F, I 



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 

 nepioiiic Sao, with two thoracic segments (Fig. 1279, B), has a head structure agreeing 

 in essential features with that in Atops or Conocoryphe (Figs. 1288, A, B). A later 

 nepioiiic stage, with eight thoracic segments (Fig. 1279, C) agrees closely with the 

 adult Ptychoparia or Olenus (Figs. 1288, C, D). These facts clearly indicate that the 

 family Oonocoryphidae should be put at the base of this extensive order. Moreover, 

 as Ptychoparia and Olenus are more primitive and simpler genera than Sao, they, as 

 typifying the family Olenidae, govern its position, which accordingly would be next 

 after the Conocoryphidae. 



Variations in the position of the eyes, the relative size of the free and fixed cheeks, 

 and the degree of specialisation of the glabella have a definite order in the ontogeny of 

 any Trilobite, and furnish characters of taxonomic value in arranging the families 

 placed under the Opisthoparia (see Fig. 1288). 



Family 1. Conocoryphidae. Angeliii. 



Free cheeks very narrow, forming the lateral margins of the cephalon, and bearing tin' 

 genal spines. Fixed cheeks large, usually traversed by an eye-line extending from near 

 the anterior end of the glabella. Facial sutures running from just within the genal 

 angles, curving forward, and cutting the anterior lateral margins of the cephalon. Eyes 



