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 in Figs. 7 and 8, 
representing Stylonurus, Slimonia, Pterygotus, Eurypterus. They 
1 
Fic. 5 (from H. Woopwarp).—1. Limulus polyphemus (dorsal aspect). 2. Limulus, 
young, in trilobite stage. 3. Prestwichia rotundata. 4. Prestwichia Birtwelli. 
5. Hemiaspis limuloides. 6. Pseudoniscus aculeatus. 
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 
