420 



THE CHANGING GENERATIONS 



resemblance ceases. The two belong to different phyla and are easily distinguished 

 by the features of the shell. In the mollusk the valves are right and left and each 

 valve is asymmetrical; in the brachiopod the valves are upper and lower, and 

 each valve is bilaterally symmetrical. Brachiopods were abundant and varied in 

 the Cambrian seas and continued to flourish throughout the Paleozoic era. Some 

 of the genera and species are important time markers for identifying Paleozoic 

 strata. From Cretaceous times onward they declined until today only a few sur- 



Fig. 27.1. Paleozoic brachiopods. A, Billingsella, Cambrian. B, Rensselaeria, Devonian. 

 C, Muerospirifer, Devonian. 



Fig. 27.2. Silurian trilobites. Left, Arctineurus; right, Bumastus. Sponges and horn or cup 

 corals are also shown. (Courtesy Rochester Museum.) 



viving types remain. One of these (Lingula, the oldest known genus of animals) 

 is remarkable in that it scarcely differs from its far distant Cambrian ancestors. 

 The trilobites (Figs. 25.2, 27.2 and B.17) were the highest form of Cambrian 

 life. They were primitive arthropods, apparently close to the ancestral stock of 

 the crustaceans. They had flattened, elongated bodies covered above with a firm 

 shell. A large anterior "head" region contained the small brain and large stomach 

 and bore two large compound eyes on the top; behind this were several or many 

 free segments and finally a tail plate which might be broad or narrow. Along the 

 top of the shell ran two grooves that divided the body into three lobes (whence 

 the name trilobite) . On the under side were two rows of legs, all alike, and running 



