286 



GEOLOGY. 



changes involved in the development of the brachiopods from the 

 theoretical primitive forms in pre-Cambrian times. Though the brach- 

 iopods were wonderfully persistent and conservative in evolution, they 

 were undergoing changes from epoch to epoch, and thus they became 

 valuable indices of progress, and serviceable means of identifying hori- 

 zons and correlating distinct deposits. 



The Cambrian Vermes. — Sea worms, or annelids, left indications of 

 their abundant presence by borings and tracks, and occasionally by 

 mud or sand heapings at the mouths of their holes (Fig. 122), but no 

 remains of the animals themselves have been found. 



The Cambrian Echinodermata. — A few cystids were present (Fig. 

 123). They had roundish or cylindrical bodies, covered unsymmet- 



rically with plates. Irregular arms 

 were attached to the upper part, 

 and a stem, often short and incom- 

 plete, was appended to the lower 

 end. The cystids were the fore- 

 runners of the beautiful crinoids 

 (stone lilies) which perhaps came 

 into existence during the Cambrian 

 times, but have not yet been found 

 in fossil form. 



The Cambrian Coelenterata. — 

 The ccelenterates were represented 

 by hydrozoa (graptolites and me- 

 c|us2e)and by anthozoa (corals), (Fig. 

 a b 124). The eccentric and almost in- 



Fig. 123. — Cambrian Echinodermata. a > explicable freaks of fossilization are 



Eocystites (?) lonqidactylus AVal(k>tt, a -. , , . m a j. i j_i 



view of the upper part of an imperfect nowhere better illustrated than in 



Cambrian cystid, showing the irregular t h e Cambrian relics of this group, 

 plates and broken arms; o, Eocystites ° L 



primcevous Billings, a single cystidian Uraptolltes, although among the 



plate * most delicate of animal forms, and 



medusae, among the softest of animals, were preserved, while much more 

 robust and indurated forms, that almost certainly were present, left 

 scant relics, or none at all. The graptolites, an extinct group, were very 

 slender, plume-like organisms, consisting of a series of cells, in which 

 the individual animal lived, attached to a common slender axis which 

 united the colony and which often branched much after the fashion of 



