298 AN INTRODUCTION TO THE STUDY OF FOSSILS 



Europe and Asia. C. niagarensis is very abundant in the 

 mid-Silurian (Niagaran) rocks of central and eastern North 

 America. 



1. Sketch specimen, labeling the various parts. 



2. Why are these found so often in an enrolled state ? 



3. Of what advantage was this to the animal ? Name a 

 living animal which enrolls. 



4. Are the eyes simple or compound ? How do you account 

 for the smooth surface ? 



5. During what eras did trilobites live? 



6. Why are the legs of trilobites found so very infrequently? 



7. Why is this genus placed under the phylum Arthropoda ? 



Phacops (Fig. 131). Silurian-Upper Devonian. 



The dorsal shield has the following characters, — {a) the 



cephalon is convex ; the glabella is very wide in front, narrowing 



B 



Fig. 131. — A trilobite. Phacops rana Green, from the Hamilton (Middle De- 

 vonian) formation of New York. A, dorsal view. Natural size. B, side view. 

 C, an eye ( X 3), showing the facets with their lenses preserved. (After Hall.) 



behind, its anterior two pairs of lateral furrows indistinct; 

 eyes very large, always with many distinct facets ; the facial su- 

 ture cuts the lateral margin of the cephalon anterior to the genal 

 angle ; {b) the thorax has eleven segments ; {c) the pygidium is 

 semi-circular. The surface of the entire dorsal shield, especially 

 the glabella, is often pustulose. The name, from the Greek 



