The Class Elasmobranchii or Shark-like Fishes 509 



The subclass Holocephali, or Chiincrras, differ from the sharks 

 in all this series of characters, and its separation as a distinct 

 group goes back to the Devonian or even farther, the earliest 

 known sharks having Httle more in common with Chimasras 

 than the modem forms have. 



The Selachii. — There have been many efforts to divide the 

 sharks and rays into natural orders. Most writers have con- 

 tented themselves with placing the sharks in one order {Squall 

 or Galei or Plenroirenii) having the gill-openings on the side, 

 and the rays in another {Raj(T, Batoidei, Hypotrema) having 

 the gill-openings underneath. Of far more importance than 

 this superficial character of adaptation are the distinctions 

 drawn from the skeleton. Dr. Gill has used the attachment 

 of the palato-quadrate apparatus as the basis of a classification. 

 The Opistharthri (Hexanchida:) have this structure articulated 

 with the postorbital part of the skull. In the Prosarthri {Hetero- 

 dontidcB) it is articulated with the preorbital part of the skull, 

 while in the other sharks (Anarthri) it is not articulated at all. 

 But these characters do not appear to be always important. 

 Chlamydoselachus, for example, differs in this regard from 

 Heptranchias, which in other respects it closely resembles. Yet, 

 in general, the groups thus characterized are undoubtedly 

 natural ones. 



Basse's Classification of Elasmobranchs. — In 1882, Professor 

 Carl Hasse proposed to subdivide the sharks on the basis of the 

 structure of the individual vertebra. In the lowest group, a 



Fig. 298. — Fin-spine of Onchus tenuistriatus Agassiz. (After Zittel.) 



hypothetical order of Polyospondyli, possibly represented by 

 the fossil spines called Onchus, an undivided notochord, perhaps 

 swohen at regular intervals, is assumed to have represented the 

 vertebral column. In the Diplospondyli {Hexanchida;) the im- 



