522 The Class Elasmobranchii or Shark-like Fishes 



Scotland, is supposed to belong to the Pleuracanthida, from 



the resemb 1 ance of the skeleton. It has no nuchal spine, and 



no trace of paired fins is preserved. 



The Cladodontida differ in having the "pectoral fin de- 

 veloped in the form of a uniserial 

 archipterygium intermediate between 

 the truly biserial one of Pleuracanthus 

 and the pectoral fin of modern sharks." 

 The numerous species are known 

 mainly from detached teeth, especially 



FIG. 3ii.-Teeth oTchdodus abundant in America, the earliest 

 striatus Agassiz. (After being in the Lower Carboniferous. One 



Davis.) Carboniferous. . _. , , . . ,_. 



species, Cladodus nelsoni (Fig. 310), 



described by Traquair, from the sub-Carboniferous of Scotland 

 shows fairly the structure of the pectoral fin. 



In Cladodus mirabilis the teeth are very robust, the crown 

 consisting of a median principal cone and two or three large 

 lateral cones on each side. The cones are fairly striate. In 

 Lambdodus from Illinois there are no lateral cones. Other genera 

 are Dicentrodus, Phcebodus, Carcharopsis, and Hybocladodus. 



