CHAPTER XV 
THE HOLOCEPHALI, OR CHIM/AERAS 
WIHE Chimeras.— Very early in geological times, cer- 
| tainly as early as the middle Silurian, the type of 
|| Chimeras diverged from that of the sharks. Hasse 
ieee them directly from his hypothetical primitive Polyo- 
spondyli, by way of the Acanthodet and Ichthyotomt. 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 Chimera 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 (oAos, whole or solid; «e@ady, 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 notochord 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- 
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