FOSSIL VERTEBRATES — FISHES 395 



Before attempting to take up the different forms of the true 

 fishes it maybe well to consider briefly those points in the anatomy 

 of fishes in general where changes have taken place resulting in the 

 modern type of the bony fishes. There are three regions in the 

 skeleton that have been used more than any others in making 

 out the different groups of the fishes and their phylogenetic 

 development : I. The gradual ossification of all the bones of 

 the bod}^ 2. The development of the vertebrae. 3. The 

 development of the fin of the modern type. 



The first of these is the gradual process of strengthening the 

 skeleton by the addition of solid matter which has been at work 

 ever since the origin of the class and is still incomplete in many 

 forms. It is only in the last sub-order of the Teleostomi, the 

 Teleocephali, that the process is at all complete. Before the 

 actual formation of bone in the supporting tissues of the body 

 the cartilage was frequently strengthened by the deposition of 

 calcareous particles. This is the condition found in the remains 

 of most of the early sharks. 



The second process, the development of the vertebrae, is of 

 considerable importance not only in the development of the 

 fishes, but as we shall see, in the earliest of the Amphibia as 

 well. The most primitive condition of the spinal column is such 

 as is well illustrated in the Amphioxus, one of the simplest of all 

 the vertebrate phylum, the column in this case consisting of a 

 continuous rod of cartilage, the notochord, extending through 

 the body from the anterior to the posterior end and lying near to 

 the dorsal side of the body. It is protected by several layers or 

 sheaths of membrane in which the future vertebrae are devel- 

 oped. The development of the bony covering of this rod is 

 foreshadowed by the appearance of the cartilaginous rings that 

 have the same serial arrangement as the vertebrae of the more 

 advanced types. A very important thing about the develop- 

 ment of the vertebrae is the development on the superior and the 

 inferior faces of the chordal sheath of bony arches that appear 

 before the body proper, or centrum, of the vertebrae. The 

 superior of these, the neural arch, protects the spinal cord 



