404 



STUDIES I'OR STUDENTS 



confined to one side of the fin, and the radials formed the othi r 

 side ; this is the type of fin present in the more advanced fishes; 

 it is called the ichthyopterygium. See big. 3. 



Fig. 3, //, archipterygium ; />', ichthyopterygium; /', basals; r, radials; '/, dermal 

 margin. 



Besides the peculiar condition of the fins, Pleuracanthus 



presents other remarkable features. Notable among these are 

 the presence of a large spine that projected from the posterior 

 edge of the skull, and the fusion of the shagreen denticles on 



the superior surface of the head into numerous derma! plates. 

 In Pleuracanthus, as well as the forms previously mentioned, the 

 center of the vertebra: had not yet become very well developed, 

 and all that is found in the preserved specimens is the line of 

 neural arches above the position of the notochord and the 

 haemal arches below. 



ChoiidrciH hclys from the Carboniferous ol Europe was in 

 many respects similar to Pleuracanthus, but there is one point of 

 decided advance, the vertebra; were well formed, though still 

 cartilaginous, and the inner part of the cartilaginous vertebral 

 ring was well filled so that the chord was divided into segments 

 by constrictions at the center of the vertebra.*. 



With the appearance of the sharks of the type of Chon- 

 drenchelys, the modern form is outlined, and since the Carbon- 

 iferous there has been no great change in the general structure 



