THE CAMBRIAN PERIOD. 



285 



trilobites, which became extinct at an early day, the brachiopods have 

 persisted to the present time, and stand as a foremost representative 



Fig. 121. — Cambrian Urachiopoda. a, Obolella gemma Billings ; b, Protor'iyncha 

 antiquata Billings, brachiopods of the Lower Cambrian; c and d, Acrotretd gemma 

 Billings, a brachiopod ranging from the Lower to the Upper Cambrian, summit and 

 side views of the pedicle or ventral valve; e, Billingsella transversa Walcott, 

 the pedicle or ventral valve of a hinged brachiopod of the Lower Cambrian. 



of extreme stability and persistence. They have changed in species 

 and, except in a few cases, in genera also, but the class has been only 

 slightly modified and still retains, with great fidelity, its distinctive 

 characteristics. 



In Cambrian times, as now, the valves of one division of the group 

 were hinged, while those of another were not. So, too, then as now, one 



Fig. 122. — Cambrian Vermes; borings and trails, a, a natural surface of sandstone 

 seen from above showing annelid borings, with mounds of sand heaped about 

 their mouths ana with trails leading away from some of them; 6, a horizontal 

 section, showing perfect borings, as well as abandoned borings whose sides have 

 been forced in during the formation of new borings, giving a crescentic cross- 

 section. Although the animal is not shown, the name Arenicolites woodi Whit- 

 field is assigned to it. 



division formed shells of calcium phosphate, and another shells of cal- 

 cium carbonate. Comparisons in other respects show that the changes 

 between the Cambrian and the present are but a small fraction of the 



