The Class Elasmobranchii or Shark-like Fishes 1 83 
The subclass Holocephali, or Chimeras, 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 little more in common with Chimeras 
than the modern 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 (Squali 
or Galet or Pleurotremi) having the gill-openings on the side, 
and the rays in another (Raje, 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 (Hexanchide) have this structure articulated 
with the postorbital part of the skull. In the Prosarthri (Hetero- 
don'ide) 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, 
‘n general, the groups thus characterized are undoubtedly 
natural ones. 
The sharks are among the earliest fishes to appear in the 
rocks, and from primitive sharks all the higher groups of 
fishes are descended. The earliest known and lowest in 
Fig. 125.—Cladoselache fyleri (Newberry), restored. Middle Devonian of Ohio. 
(After Dean.) 
structure constitute the order Pleuropterygii (Cladoselachide), 
typified by Cladoselache fyleri from the middle Devonian of 
Ohio. 
