THE ORDOVICIAN PERIOD. 



349 



Fig. 157. — Ordovician Trilobites. a, Trinucleus ornatus Sternberg; b, Harpes primus 

 Barrande; c, Bronteus lunatus Billings; d, Pterygometopus callicephalus (Hall); 

 e, Odontopleura crosotus (Locke); /, lllcenus americanus Billings; g, Proetus 

 parviusculus Hall; h, Isotelus maximus Locke; i-j, Calymene callicephala Green; 

 k, Ceraurus pleurexanthemus Green; Z, Lichas incola Barrande; m, Bumastus 

 trentonensis (Emmons). The lowest order of Trilobites, the Hypoparia, are rep- 

 resented by figures a and b. They have no true eyes, and the facial suture is 

 marginal or sub-marginal. Figures e, /, g, h, I, and in are examples of the trilo- 

 bite order Opisthoparia, in which the posterior limb of the facial suture is directed 

 backward, cutting the posterior margin of the head and leaving the genal angle 

 on the free cheek. The different degrees of differentiation of the glabella should 

 be noted. Figures d, %-], and k are examples of the order Proparia which includes 

 the highest of the trilobites; the posterior limb of the facial suture is directed 

 laterally, leaving the genal angle upon the fixed cheek; this order was not present 

 in the Cambrian faunas. 



