372 THE CAMBRIAN PERIOD 



brian ; certain fossils which by some authorities are called corals 

 are by others regarded as sponges. At all events, they are not 

 conspicuous elements in the fauna. 



Echinodermata. — The Echinoderms are rare and are prin- 

 cipally Cystids, a very primitive grade of the type ; true Crinoids 

 and Star-fishes appear before the close of the period. 



Worms. — The presence of marine worms is indicated by tracks 

 and borings in the sands which have now consolidated into hard 

 rocks. 



Arthropoda. — The only known Cambrian Arthropods are the 

 Crustacea, and of these much the most abundant group is that of 

 the Trilobita, which are altogether confined to the Palaeozoic 

 rocks and are by far the most important of Cambrian fossils. It 

 is only within recent years that the systematic position of the 

 Trilobites has been established through the fortunate discovery 

 of specimens with their appendages attached (see Fig. 138). Tri- 

 lobites have a more or less distinctly three-lobed body, at one end 

 of which is the head-shield, usually with a pair of fixed compound 

 eyes ; at the other end is the tail-shield, and between the two 

 shields is a ringed or jointed body made up of a variable number 

 of movable segments. The Trilobites display an extraordinary 

 variety in form and size, in the proportions of the head and tail- 

 shields, in the number of free segments, and in the development of 

 spines. Already in the Cambrian this wealth of forms is notable, 

 though far less so than it became in the Ordovician. As compared 

 with those of later times, the Cambrian Trilobites are marked by 

 the (usually) very small size of the tail-shield, the large number of 

 free segments, and their inability to roll themselves up. Some of 

 them, like Paradoxides, are very large (from 10 inches to 2 feet 

 in length). Olenellus also has large species, while Agnostus is ex- 

 cessively small and without eyes. 



The great importance of the Trilobites for Cambrian stratigraphy 

 is indicated by the fact that the three divisions of the system 

 are named for the three dominant genera of these crustaceans, 

 Olenellus, Paradoxides, and Olenus. 



Two other divisions of the Crustacea are found in the Cam- 



