76 



HISTORICAL GEOLOGY 



They were inhabitants of the sea and they were the most highly 

 organized animals of the time. Trilobites persisted only till the 

 close of the Paleozoic era, and were especially numerous in the 

 earlier periods of that era. The name Trilobite refers to the three- 

 lobed character of the body. The creature possessed a distinct 

 head-shield with compound eyes, and a more or less distinct tail- 

 shield. Between the shields there 

 was a highly segmented body por- 

 tion. They ranged in length from 

 an inch or less to about two feet. 

 "The Trilobites display an extraor- 

 dinary variety in form and size, 

 in the proportion of the head-and- 

 tail-shields, in the number of free 

 segments, and in the development 

 of spines. Already in the Cambrian 

 this wealth of forms is notable, 

 though far less than it became in 

 the Ordovician. As compared with 

 those of later times, the Cambrian 

 Trilobites are marked by the (usu- 

 ally) very small size of the tail-shield, 

 the large number of free segments, 

 and their inability to roll them- 

 selves up." l Eucrustaceans of rather 

 simple types (e.g. Ostracods) were 

 present, but not important. 

 Arachnids (e.g. Eurypterids) are only sparingly known from 

 the Cambrian. These forms will be discussed beyond. 



No Known Land Animals or Vertebrates. — Thus far no 

 fossil land animals of any kind have been found, but this by no 

 means proves their non-existence during the Cambrian. Ver- 

 tebrates are entirely unknown, and if any existed in the Cambrian 

 we know, from our study of Vertebrates of succeeding periods, 

 that they must have been of the very simplest types. It is exceed- 

 ingly doubtful if any existed. 



1 W. B. Scott: An Introduction to Geology, 2nd Ed., p. 556. 



Fig. 40 

 A Middle Cambrian Trilobite, 

 Neolemus serratus, with well 

 preserved appendages. (After 

 Walcott.) 



