THE CAMBRIAN PERIOD 



497 



outwards. In later epochs, these crescents became more and more 

 curved, extending the sweep of vision fore and aft, to the trilobite's 

 obvious advantage. 



The upper surface of the body was ornamented variously with 

 granules, spines, and other markings, the significance of which is 

 little understood. These ornamentations varied as time went on, 

 increasing, in general, until after the climax of the trilobites had 

 been passed. Trilobites possessed a row of slender articulated 



Fig. 373. Cambrian Brachiopods: a and 6, Acrotreta gemma Billings, a 

 brachiopod ranging from the Lower to the Upper Cambrian, summit 

 and side views of the ventral valve; c, Billingsella transversa Walcott, 

 a pedicle or ventral valve of a hinged brachiopod of the Lower Cam- 

 brian; d and e, Lingulepis pinifprmis Owen, views of the two valves; 

 /, and g, Kutorgina cingulata Billings, side and dorsal or brachial views; 

 h, Billingsella coloradoensis (Shum.). 



limbs on either side, and delicate filaments which served the function 

 of respiratory organs. The nature of the limbs indicates that the 

 trilobites both walked and swam. They possessed antennae which 

 doubtless served as organs of touch, and they moulted the shell at 

 successive stages of growth, like modern crabs. Omitting further 

 details, it is to be observed that, at this early day, a highly complex, 

 well-differentiated organization had been acquired, possessing 

 nearly all the organs and functions of arthropods of the present day. 

 Molluscoidea. This branch was well represented by brachiopods 

 (lamp-shells, Fig. 373). In geological importance, the brachiopods 



