CHAPTER XXXI 



THE HOLOCEPHALI, OR CHIMERAS 



HE Chimaeras. — Very early in geological times, cer- 

 ^. tainly as early as the middle Silurian, the type of 

 Chiiiiccras diverged from that of the sharks. Hasse 

 derives them directly from his hypothetical primitive Polyo- 

 spondyli, by way of the Acanthodei and Ichthyotomi. In any 

 event the point of divergence must be placed very early in the 

 evolution of sharks, and this suggestion is as likely as any other. 

 The chief character of Chimeras is found in the autostylic skull, 

 which is quite different from the hyostylic skull of the sharks. 

 In the sharks and in all higher fishes the mandible is joined to the 

 skull by a suspensorium of bones or cartilages (quadrate, sym- 

 plectic, and hyomandibular bones in the Teleost fishes). To this 

 arrangement the name hyostylic is given. In the Chimsera there 

 is no suspensorium, the mandible being directly attached to 

 the cranium, of which the hyomandibular and quadrate elements 

 form an integral part, this arrangement being called autostylic. 

 The palato-quadrate apparatus, of which the upper jaw is the 

 anterior part, is immovably fused with the cranium, instead 

 of being articulated with it. This fact gives the name to the 

 subclass Holocephali {o\oz, whole or solid; KecpaX?'/, head). 

 Other characters are found in the incomplete character of the 

 back-bone, which consists of a scarcely segmented notochord 

 differing from the most primitive condition imagined , only 

 in being surrounded by calcareous rings, no lime entering into 

 the composition of the i.otochord itself. The tail is diphycercal 

 and usually prolonged in a filament (leptocercal). The shoulder- 

 girdle, as in the sharks, is free from the skull. The pectoral 

 fins are short and broad, without segmented axis or archiptery- 



cfii 



