THE CAMBRIAN PERIOD. 



283 



trilobites rolled themselves up armadillo fashion — another mark of 

 progress. The upper surface of the body was, even in Cambrian times, 

 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. 



The understructure is not revealed in the Cambrian specimens, 

 the parts having been lost in fossilization, but later forms, worked out 

 by Walcott, Beecher, and others, show that the trilobites possessed a 

 row of slender articulated limbs on either side, and also delicate filaments 

 which served the function of respiratory organs. The nature of the 

 limbs indicates that the trilobites both walked and swam. It is inferred 

 that some were swift of movement, while the weakness of the ambu- 

 latory organs in others implies that they were slow. The trilobites 

 also possessed antennae which doubtless served as organs of touch — 

 another indication of the definite development of the senses, even at 

 this early time. It is interesting to note that the habit of moulting 

 the shell at successive stages of growth had been acquired. Leaving 

 further details to the special 

 study of the accompanying 

 figures, it is to be observed that, 

 at this early day, there had been 

 acquired a highly complex, well- 

 differentiated organization, en- 

 dowed with nearly all the organs 

 and functions possessed by sim- 

 ilar crustaceans of the present 

 day. 



Besides these ancestral crus- 

 taceans, a few other forms of 

 interest were present, among 

 which were little ostracodes which 

 had valve-like shells on their 

 backs much like those of bivalve 

 mollusks (Fig. 119, c), an ancient type of phyllopods represented by 

 the singular form Protocaris (Fig. 119, d), and a phyllocarid (Fig. 119, a 

 and b). 



The Cambrian Mollusca. — A few cephalopods (chambered shells) 



b c ' 'd 



Fig. 119. — Other Cambrian Crustacea, a, 

 Hymenocaris vermicauda Salter; b, Aristo- 

 zoce rotundata Walcott, a Cambrian phyllo- 

 carid; c, Leperditia dermatoides Walcott, a 

 Cambrian ostracode; d, Protocaris marshi 

 Walcott, a Cambrian phyllopod. 



