52: 



The Class Elasmobranchii or Shark-like Fishes 



Scotland, is supposed to belong to the Pleuracanthidw, from 

 the resemblance of the skeleton. It has no nuchal spine, and 

 no trace of paired fins is preserved. 



The Cladodontidm differ in having the "pectoral fin de- 

 veloped in the form of a uniserial 

 archipterygium intermediate between 

 the truly biserial one of Pie nr acanthus 

 and the pectoral fin of modern sharks." 

 The numerous species are known 

 mainly from detached teeth, especially 

 abundant in America, the earliest 

 being in the Lower Carboniferous. One 

 species, Cladodiis nelsoni (Fig. 310), 

 described by Traquair, from the sub -Carboniferous of Scotland 

 shows fairly the structure of the pectoral fin. 



In Cladodiis iiiirabilis 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 

 Lanibdodiis from Illinois there are no lateral cones. Other genera 

 are Dicciitrodns, Phwbodus, Carcliaropsis, and Hyhocladodiis. 



Fig. 3U.— Teeth of Cladodiis 

 striatiix Agassiz. (After 

 Davis.) Carboniferous. 



