336 



HISTORICAL GEOLOGY 



FIG. 330. a and b. 

 brian brachiopod. 



A Cam- 

 Interior 



and exterior views of the shell. 



are commonly ornamented only by concentric lines of growth 



(Fig. 330). Internally they exhibit the simplest type of 



brachiopod structure, the spiral 

 feeding arms not supported by 

 hard skeletons, and hence not 

 preserved, and the two shells held 

 together by muscles only, rather 

 than by a solid hinge. 



The trilobites had attained 

 somewhat greater variety of form 



even before the Cambrian period began, 



and were seemingly more advanced in 



their cycle of evolution. Some very 



simple types (Figs. 331 and 332) were 



present, species which were eyeless and 



had only two body segments between the 



broad head and tail. Others were of large 



size (even exceeding two feet in length, in 



exceptional instances) and were ornamented 



with spines and raised lines (Figs. 333 and 



334). Most of them possessed prominent 



compound eyes not unlike those of insects, 



and they were provided with a generous 



number of jointed legs of a type adapted 



to swimming. These crustaceans, by vir- 

 tue of their advantage in size and their 



greater intelligence and activity, doubtless 



held the dominant place in the animal 



world of their day. Many other groups, 



such as the corals, mollusks (Figs. 335 and 



336), worms, and graptolites, have left 



representatives among the fossils of the 



Cambrian strata, but they scarcely attained 



prominence until later periods. As yet we have no knowledge 



of the existence in the Cambrian of air-breathing animals, such 



as insects, nor of even the simplest vertebrates. 



FIG. 331. One of 

 the earliest and 

 simplest trilobitca 

 (Agnostus), char- 

 acteristic of the 

 Cambrian rocks. 



FIG. 332. A larger 

 Cambrian trilobite 

 (Conocoryphe). 



Compare this 

 with Silurian va- 

 rieties. In which 

 are the eyes visible ? 



