558 POPULAR SCIENCE MONTHLY. 



a word once used in the widest fashion for all sorts of mailed fishes, 

 but little by little restricted to the hard-scaled relatives and ancestors 

 of the gar-pike of to-day. 



Dimly seen in the vast darkness of Paleozoic time are the huge 

 figures known as Arthrognaths. These are mailed and helmeted fishes, 

 limbless so far as we know, but with sharp, notched, turtle-like jaws 

 quite different from those of the fish or those of any animal alive to-day. 

 These creatures appear in Silurian rocks and are especially abundant 

 in the fossil beds of Ohio, where Newberry, Claypole, Eastman and 

 Dean have patiently studied the broken fragments of their armor. 

 Most of them have a great casque on the head with a shield at 

 the neck and a movable joint connecting the two. Among them was 

 almost every variation in size and form. 



These creatures have been often called ganoids, but with the true 

 ganoids like the gar-pike they have seemingly nothing in common. 

 They are very different from the Ostracophores. To regard them as 

 derived from ancestral Dipnoi is to give a possible guess as to their 

 origin and a very unsatisfactory guess at that. But they have all 

 passed away in competition with the scaly fishes and sharks of later 

 evolution, and it seems certain that they, like the mailed Ostracophores, 

 have left no descendants. 



Next after the lampreys, but a long way after in structure as in 

 time, come the sharks. With the sharks appear for the first time true 

 limbs and the lower jaw. The upper jaw is still formed from the palate, 

 and the shoulder girdle is attached behind the skull. "Little is known," 

 says Professor Dean, "of the primitive stem of the sharks and even the 

 lines of descent of the different members of the group can only be 

 generally suggested. The development of recent forms has yielded few 

 results of undoubted value to the phylogenist. It would appear is if 

 paleontology alone could solve the puzzles of their descent." 



Of the very earliest sharks in the Upper Silurian age the remains 

 are too scanty to prove much save that there were sharks in abundance 

 and variety. Spines, teeth, fragments of shagreen, show that in some 

 regards these forms were highly specialized. In the Carboniferous age, 

 according to Dean, 'there occurred the culminating point of their differ- 

 entiation when specialized sharks existed, whose varied structures are 

 paralleled only by those of the existing bony fishes, — sharks fitted to 

 the most special environment, some minute and delicate, others enor- 

 mous, heavy and sluggish, with stout head and fin spines and elaborate 

 types of dentition.' 



The sharks are, however, doubtless evolved from the still more 

 primitive shark without limbs and with the teeth slowly formed from 

 modification of the ordinary shagreen prickles. In determining the 

 earliest among the several primitive types of shark we are stopped by 



