CHAPTER XXX 



THE TRUE SHARKS 



RDER Notidani. We may recognize as a distinct 

 order, a primitive group of recent sharks, a group of 

 forms finding its natural place somewhere between 

 the Cladoselachida and Heterodontidce, both of which groups 

 long preceded it in geological time. 



The name Notidani (Notidanus, voonddvo?, dry back, an old 

 name of one of the genera) may be retained for this group, 

 which corresponds to the Diplospondyli of Hasse, the Opis- 

 tharthri of Gill, arid the Protoselachii of Parker and Haswell. 

 The Notidani are characterized by the primitive structure of 

 the spinal column, which is without calcareous matter, the centra 

 being imperfectly developed. There are six or seven branchial 

 arches, and in the typical forms not in Chlamydoselachus) the 

 palato-quadrate or upper jaw articulates with the postorbital 



FIG. 312. Griset or Cow-shark, Hexanchus griseus (Gmelin). Currituck Inlet, N. C. 



region of the skull. The teeth are of primitive character, of 

 different forms in the same jaw, each with many cusps. The 

 fins are without spines, the pectoral fin having the three basal 

 cartilages (mesopterygium with propterygium and metapte- 

 rygium) as usual among sharks. 



The few living forms are of high interest. The extinct species 

 are numerous, but not very different from the living species. 



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