PALEOZOIC ROCKS AND ERA. 



283 



Orthoceratites were extremely abundant in Silurian 

 times, and, in some cases, reached an enormous size. Jn 

 the Silurian of the Western States, specimens have been 

 found which were eight to ten inches in diameter, and 

 over fifteen feet long. They were the most formidable 

 animals of these early seas. They came in with the Pri- 

 mordial, reached their maximum in the Mid-Silurian, but 

 continued through the Paleozoic, and then passed away 

 forever. 



Although the straight, chambered shells were by far 

 the most abundant, yet the coiled kinds were also found. 



Crustacea ; Trilobites. Passing over the worms, as 

 being of less importance, although their borings, their 

 tracks, their calcareous tubes, and even their teeth, have 

 been found, we come at once to perhaps the most abun- 

 dant and characteristic of all Paleozoic forms Trilobites. 



Description. The shell of these curious creatures 

 was convex above and* flat or, more probably, concave 

 below (Fig. 196). It was divided, like most crustaceans, 

 into many movable joints, but several front joints were 

 always consolidated to make a buckler, or head-shield, 

 and usually, but not always, several hind joints were con- 

 solidated to form a pygidium, or tail-shield. Longitudi- 



FIG. 195. Structure of the eye of trilobites: a, Dalmania pleuropteryx; , eye slightly 

 magnified; c, eye more highly magnified. (After Hall.) 



