THE STATE GEOLOGIST. 39 



and the highest types of animal life consisting of forms related 

 to the scorpion and king-crab. "Monsters in those days" there 

 were none; life, such as it was, existed in profusion, but was of 

 decidedly inferior organization, sluggish or sessile, mostly of 

 small size, and rather uniformly distributed. But already, at as 

 far distant a period as the oldest fossiliferous horizon, differentia- 

 tion had been taking place, and all the great divisions of inverte- 

 brates had become definitely established. Finally it came to 1 pass, 

 in some manner and at some epoch — how and when we know not 

 for certain — that the earliest chordate animals were introduced; 

 that is to say, animals ancestral to' modern vertebrates, probably 

 cartilaginous and with only dermal folds for limbs, but craniate, 

 and having an axial skeleton. 



Some have imagined that the transition from invertebrates to 

 chordates occurred through annelid worms, others through 

 jointed animals (Arthropods), but here at least is a great gap as 

 yet unfilled. All that we can affirm is that the Cambrian system 

 has yielded hitherto no trace of forms which one may regard as 

 standing in ancestral relations to* chordates, and it is not until the 

 Ordovician (or Lower Silurian) that we first meet with such 

 creatures in the reality. These primitive, weird-looking organ- 

 isms differ from fishes proper, and likewise from all other verte- 

 brates, in the absence of paired limbs and of a lower jaw, as well 

 as in the microscopical structure of their hard parts. Under the 

 name of Ostracophores (literally "shell-bearing"), Professor 

 Cope has placed them in a distinct class (Agnatha) , thus sharply 

 separating them from fishes proper. Nevertheless they approach 

 in other respects very closely to fishes, and when we remember 

 that the great group of Elasmobranchs (sharks and rays) has 

 equally remote an origin, it will be clear that the history of verte- 

 brate life on our globe extends over incredibly long periods of 

 time. 



One of the best known of these primitive vertebrates is that 

 curious form to which Agassiz has given the name of Pterichthys, 

 familiar to all readers of Hugh Miller's fascinating works. The 

 first impression produced by these bizarre creatures upon the 

 mind of their discoverer has been graphically described both by 

 Miller and Agassiz. Says the latter: "This remarkable animal 



