34 EVOLUTION IN THE PAST 



fashion ; and the latter seem already to have been on the 

 decHne. 

 TRILOBITES Trilobites continued in considerable force, and were widely 

 distributed. The bUnd forms were still languishing, and 

 reverses had been sustained by several old families gifted 

 with eyesight. In some genera important changes were 

 taking place in regard to the eyes. In certain cases these 

 organs had been shifted to new positions ; and some forms 

 bore their eyes elevated on stalks. There was apparently more 

 need for vigilance than in times of yore. 



Many of the trilobites had undergone variations in the shape 

 of their head and tail armour (Phacops, Sphcerexochus, etc.) ; 

 and the power to roll up in wood-louse fashion was still more 

 in evidence. Many of the modifications seem to have been 

 the result of a mere exuberance of energy : at any rate their 

 precise use and value cannot be determined. Fantastic forms 

 were now to be seen. Some of these had swollen unchecked 

 heads, covered with pimple-like excrescences (Staurocephalus). 

 In some species the cheek armour had been modified into 

 spines, suggestive of rigid fins ; and the tail armour had 

 followed suit (Deiphon). Several forms were appearing with 

 long spiky taUs, enabling an animal thrown on its back to 

 rectify its position {Dalmanites limulurus, etc.). Trilobites, 

 however, though still able to adapt and re-adapt themselves 

 to the pressure of conditions, were coming to the end of their 

 resources ; and during this Period their decline commenced. 

 KING-CRABS Several species of small animals with head-shields and 

 spike-hke tails, and holding affinities with trilobites and 

 king-crabs were now on the scene. Animals with these 

 double affinities had appeared as early as the Cambrian 

 Period {Aglaspis). The Silurian forms were certainly more in 

 Une with king-crabs. They retained, however, a marked 

 trilobite character, as the segments of the middle body or 

 thorax were not fused together : the second shield of the 

 king-crab was, therefore, undeveloped {Hemiaspis limuloides, 

 Woodw., Neolimulus falcatus, Woodw.). King-crabs, in 

 short, were as yet but half made. 

 SEA- Sea-scorpions — of small account in Ordovician times — 



SCORPIONS were now giving promise of their coming importance. As 



