THE CAMBRIAN PERIOD 



415 



shells and well-developed hinges and dissimilar valves. 

 Of these two subdivisions the first and most primitive 

 was more abundant in the Cambrian, and the second, 

 later in the Paleozoic. In the Lower Cambrian 22 

 genera of brachiopods have been found in Europe and 

 North America, showing that the class was probably 

 well-developed in the preceding era (Proterozoic). 

 The two subdivisions of brachiopods are living in the 

 seas of the present, having undergone many changes 

 during their long existence; yet the class as a whole 

 has been little modified since Cambrian times. syO>£) I 



Echinodermata 



Cystoids (Fig. 385) of very simple structure lived in 

 the Cambrian, and sea cucumbers have been discovered 

 in Middle Cambrian strata in British Columbia. 



Fig. 385.— 

 Cambrian cys- 

 toid : Eocystites 

 longidactylus. 



Worms 



Perhaps the fossils next in abundance to the trilobites are the trails 

 and borings of worms. In certain Lower Cambrian beds vertical 

 .,..—__ tubes (Scolithus) are so common as to give a 



^u ^ striped appearance to the rock. Many fossils 



which were at one time thought to be fossil 

 marine plants are now known to be the trails 

 of worms, or borings which have been filled 

 with sand or clay. Since worms are, as a rule, 

 destitute of hard parts, it is seldom that any 

 traces of the actual animals have been pre- 

 served. In some fine shales of the Middle 

 ^^ B M Cambrian in British Columbia, however, the 

 fleshy parts of the animal are sometimes pre- 

 served as a glistening surface, even to the fine 

 details of the structure (Fig. 386 A, B). 



X' 1 



*W 



Fig. 



386.- 

 worms : A, Ottoia pro- 

 lific a; B, Worthenella 

 cambria. These fossils 

 are represented by a 

 thin film which is darker 

 than the shale contain- 

 ing them, the contents 

 of the animal being pre- 

 served as a glistening sur- 

 face. (After Walcott.) 



' CXELAND GEOL 



Ccelenterata 



Corals. — Cambrian corals (Fig. 387) were 

 so simple in structure that some of them have 

 been called sponges by certain writers and corals 

 by others. This group was abundant locally, 

 but in general was rare throughout the period. 

 — 27 



