THE ORDOVICIAN PERIOD. 



351 



The eyes of the Ordovician trilobites, as a rule, were more prominent 

 and better developed than those of the Cambrian forms, and showed 

 improved adaptation by being set on a curved pedestal, so that some 

 eyelets were directed more to the front and others more to the rear, giv- 

 ing an ampler angle of vision (see particularly d and h, Fig. 157). There 

 was little or no increase in the average size of the trilobites. The 

 Isotelus gigas sometimes reached a length of 18 inches and ranked 

 among the giant trilobites, but it was equaled and even surpassed 

 by some of the great Paradoxides of the Cambrian. Progress took 

 the form of improvement of the parts and of superior adaptation rather 

 than increased size. Curious ornamentation of the exterior was dis- 

 played in some genera, as were also eccentric departures from the 

 simple three-lobed form, but such aberrations became far more pro- 

 nounced in the early stages of the decline of the trilobites, in later 

 periods. 



Other crustaceans. — Besides the trilobites, the crustaceans were 

 represented by a few inferior forms. Ostracodes were common in 



Fig. 159. — Ordovician Crustacea, other than Trilobites. a-b, Ceratopsis oculifera 

 (Hall), lateral and ventral views; c, Eurychilina reticulata Ulrich; d, Macronotella 

 scofieldi Ulrich; e, Ceratopsis chambersi (Miller); f-g, Tetradella quadrilirata 

 H. and W., lateral and dorsal view; h, Aparchii.es minutissimus Hall; i-j, 

 Lepidocoleus jamesi (Meek), a single plate and a complete individual. Figures 

 a-b, c, d, e, f-g, and h are examples of Ordovician ostracods, all of them much 

 magnified. Figures i-j are illustrations of a single plate and of the complete 

 individual of a cirriped crustacean. 



some beds (Fig. 159, a, b, c, d, e, /, g, and h) and there were a few cirripeds 

 of which the barnacle is the most familiar modern type; but, unlike 

 the barnacle, the Ordovician cirripeds had an elongate body and were 

 not sessile (Fig. 159, i, j). The euripterids, a limuloid form related 

 to the arachnids, appeared in the later part of the period. 



The dominance of the cephalopods. — The largest, the most powerful, 

 and perhaps the most predaceous of the known forms of Ordovician 



