626 ARTHROPODA 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 sutwres 
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 strueture of the cephalon with the 
genera of the preceding order. Therefore this stage does not enter into consideration 

Cephala of Opisthoparia. A, Atops. B, Conocoryphe. C, Ptychoparia. D, Olenus. E, Asaphus. F, Ilaenus. 
G, Prottus. H, Bronteus. I, Lichas. J, Acidaspis (after Beecher). 
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 (Fig. 1279, B), has a head structure agreeing 
in essential features with that in Atops or Conocoryphe (Figs. 1288, A, B). <A later 
nepionic 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 Conocoryphidae 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, Angelin. 
Free cheeks very narrow, forming the lateral margins of the cephalon, and bearing the 
genal spines. Fixed cheeks large, uswally 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 
— = 
