THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 25 



of animals which existed at this period — the giant sea-scorpions, or 

 Gigantostraca. This group was closely associated with the king- 

 crabs, and the two groups together are classified under the title 

 Merostomata. 



The appearance of these sea-scorpions is given iti Figs. 7 and 8, 

 representing Stylonurus, Slimonia, Pterygotus, Eurypterus. They 



Fig. 5 (from H. Woodward). — 1. Limulus polyphemus (dorsal aspect). 2. Lunulas, 

 young, in trilobitc stage. 3. Prestwichia rotundata. 4. Prestivichia Birtwelli. 

 5. Hemiaspis limuloides. 6. Pseudoniscus acitlcatus. 



must have been in those days the tyrants of the deep, for specimens 

 of Pterygotus have been found over six feet in length. 



At this time, then, by every criterion hitherto used, by the 

 multitude of species, by the size of individual species, which at this 

 period reached the maximum, by their subsequent decay and final 

 extinction, we must conclude that these forms were in their zenith, 

 that the predominant race at this time was to be found in this group 

 of arthropods. Just previously, the sea swarmed with trilobites, and 

 right into the period when the Gigantostraca flourished, the tril obites 



