THE CAMBRIAN PERIOD. 279 



surplus might not be worth noting if there were not specific reasons 

 for regarding it as large. Not a few of the Cambrian animals were 

 fixed to the bottom of the sea, and hence there must have been organic 

 matter enough floating in the water to bring them their daily food, 

 and this implies a very general enrichment of the oceanic waters with 

 plants. Probably these were largely minute one-celled plants of the 

 algae type, and hence they easily escaped fossilization. The inadapta- 

 bility of the lower plants to fossilization is doubtless the chief explanation 

 of the poor representation of the plants among the Cambrian fossils. 



Reasons of a physical nature have previ- 

 ously been given (p. 217) for thinking that the 

 surface of the land was clothed with some kind 

 of vegetation, and this is an additional reason 

 for assuming a very general, if not nearly uni- 

 versal, distribution of plants over the surface 

 of the globe. But there are no identifiable 

 traces of land-plants, and but very obscure im- 

 pressions of sea-plants. There are some ill-de- 

 fined stem-like or frond-like impressions, com- 

 monly styled "fucoidal," that may be, in part 

 at least, the casts of sea-weeds. A peculiar 

 form consisting of clusters of radiating rays _ 



. ° J Fig. 117. — Oldhamia anhqua 



(Oldhamia, Fig. 117) is found in abundance in Forbes, a problematical 

 Cambrian rocks in Ireland, and has been referred reffrre^t^^armT^l^ 

 to algae, but the identification is not beyond |™t it may possibly be- 

 question. These and a few other doubtful 



forms make up the whole record as thus far revealed. The lesson 

 taught is the extreme imperfection of the fossil record. 



The Animal Fossils. 



Turning to the record of animal life, it appears by contrast that 

 every great division of the animal kingdom, except the vertebrate, 

 had its representatives in Cambrian times. The Arthropoda were rep- 

 resented by crustaceans; the Mollusca, by cephalopods, gastropods, 

 pteropods, and pelecypods ; the Molluscoidea, by brachiopods and 

 bryozoans; the Vermes, by annelids; the Echinodermata, by cystoids; 

 the Coelenterata, by graptolites, medusae, and corals; the Porifera, by 



