118 



HISTORICAL GEOLOGY 



had little or no real value in their life activities. We shall see in 

 studying later periods that similar eccentricities mark the fall of 

 other groups, such as the Ammonites and the Reptiles. " l Eucrus- 

 taceans were much like those of the Ordovician. 



Arachnids, represented by the Eurypterids, greatly increased 

 in numbers, species, and size, and they appear to have culmi- 

 nated in this period. The 

 following brief description, 

 together with an examina- 

 tion of Fig. 65, will serve 

 to give a fair idea of the 

 appearance and structure 

 of these remarkable crea- 

 tures. In the typical 

 Eurypterid, a quadrate or 

 semicircular head has be- 

 hind it twelve movable 

 segments making up the 

 abdomen, and attached to 

 the last segment is either 

 a spine or plate-like tail. 

 The five pairs of append- 

 ages all come out from the 

 head portion, thus being 

 markedly different from 

 the Trilobites. The first 

 pair of appendages are 

 much enlarged, sometimes 

 provided with pincers and 

 sometimes not, while the 

 fifth pair are usually long 

 and they serve as swim- 

 ming paddles. They varied greatly in size, one species, from the 

 Silurian, having attained a length of over six feet and so is one of 

 the largest known Arthropods. Many Eurypterids appear to have 

 been marine animals, while others probably lived in fresh or brack- 

 ish water lagoons. The Arachnids included also the earliest known 

 Scorpions (Fig. 66), which were in many respects similar to the 

 Eurypterids. 



1 Blackwelder and Barrows: Elements of Geology, pp. 352-353. 



Fig. 67 

 A bit of Silurian sea-bottom showing 

 Crinoid, Bryozoan, Brachiopod, and 

 Trilobite remains. (W. J. Miller, photo.) 



