3<5 



CAMBRIAN PERIOD 



cents became more and more curved, extending the sweep of vision 

 fore and aft, to the animal's obvious advantage. 



The upper surface of the body was ornamented variously, and 

 the ornamentation 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 legs on either side, and deli- 

 cate filaments which served the function of respiratory organs. 

 The nature of the legs indicates that 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 organi- 

 zation had been acquired, possessing nearly all the organs and 

 functions of arthropods of the present day. 



Brachiopods (molluscoidea, p. 686 and Fig. 324) were second in 

 geological importance to trilobites; but unlike trilobites, brachiopods 

 still live. They are conspicuous representatives of stability and 

 persistence. Though the species and most of the genera have 



Fig. 324. CAMBRIAN BRACHIOPODS: a and b, 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 braehiopod of the Lower Cambrian; d and e, Lingukpis pinni- 

 formis Owen, views of the two valves;/ and g, Kutorgina cingulata Billings, side and 

 dorsal or brachial views, a Lower Cambrian species; h, Billingsella coloradocnsis 

 (Shum.), an Upper Cambrian species. 



changed, the class as a whole has been but slightly modified since 

 the Cambrian period. The brachiopod shell is bivalve. The two 

 valves are unlike, but each is bilaterally symmetrical (Fig. 324). 



