PALEOZOIC ROCKS AND ERA. 291 



In some a chirping organ has been found. This shows 

 that an auditory apparatus was already developed. 



Although these first known insects are among the lower 

 orders of the class, and also are connecting links between 

 two such lower orders, yet their somewhat perfect devel- 

 opment indicates that we must look for the very first 

 insects still lower, i. e., in the Upper Silurian. They 

 have been recently found there. 



Fishes. The introduction of fishes must be regarded 

 as a great step in the progress of life, for it is the begin- 

 ning not only of a new and higher class, but of a new 

 great department and the highest, viz., Vertebrata. They 

 commenced first in the lowest Devonian or perhaps even 

 in the uppermost Silurian, few in numbers, small in size, 

 and of strange, un-fish-like forms, but soon developed 

 in size and numbers until these early seas swarmed with 

 them, and they quickly became the rulers of the age. 

 The previous rulers, Orthoceratites and Trilobites, there- 

 fore diminish in number and size, and thus seek safety in 

 subordination. As examples of the great size of Devonian 

 fishes, we mention a few. The Onchyodus (claw-tooth) 

 had jaws eighteen inches long, armed with teeth two or 

 more inches long ; the Dinichtliys (huge fish) was fifteen 

 to eighteen feet long, three feet thick, and had jaws two 

 feet long, armed with curious blade-like teeth. These 

 are from America. The Asterolepis (star-scale) of Europe 

 is believed to have been twenty feet long, and of propor- 

 tionate dimensions. 



We must not imagine, however, that these fishes were at 

 all like most common fishes of the present day. Neglect- 

 ing some rare and unusual kinds, fishes may be divided 

 into three great orders, viz., 1. Teleosts (complete bone); 

 2. Ganoids (shining) ; and 3. Elaxniobrauclis (plate-gills). 

 The Teleosts include all the ordinary fishes : examples of 

 Ganoids are found in gar-fish and sturgeon ; of Elasmo- 

 branchs, in sharks, skates, and rays. At the present 



