Animals and Plants of Past and Their Records 609 



Fig. 317. — Specimens showing a natural mold, A, of the interior of an animal 

 from which the shell has disappeared ; B, the original shell of a similar speci- 

 men. (From Cleland: Physical and Historical Geology, published by the Amer- 

 ican Book Company.) 



Fig. 318. — Trilobite, an extinct, fossil marine arthropod of the Ordovician 

 period (see Figs. 320-322). Dorsal (left) and ventral views show the restored 

 appendages. Trilobites (Gr. tri, three; lobos, lobes) had flattened, oval bodies 

 composed of head, thorax, and abdomen; on the dorsal side the body is divided 

 lengthwise by furrows into 3 lobes (one median and two lateral). They possess 

 one pair of delicate antennae; their gills are attached to thoracic appendages; they 

 have numerous, delicate, biramous appendages. (From Cleland: Physical and 

 Historical Geology, published by the American Book Company.) 



