568 PISCES—FISHES. 
gills) and Heptanchus (seven gills), the mouth is almost inferior, the 
vertebral column is imperfectly segmented with persistent notochord. 
History.—The Elasmobranchs appear in the Upper Silurian, are 
very abundant from the Carboniferous onwards, but are now greatly 
outnumbered by the Bony Fishes. An increasing calcification of the 
axial skeleton is traceable through the ages, and in some of the 
ancient forms the exoskeleton was greatly developed, often including 
long spines or ichthyodorulites firmly fixed on the dorsal fins or on 
the neck. 
Order 2, HOLOCEPHALI 
The Holocephali are represented by the sea-cat or Chimera from 
northern seas, and Callorhynchus from the south. There is a fold or 
operculum covering the (4) gill-clefts and leaving only one external 
opening on each side; there is no spiracle; the vertebral column is 
unsegmented ; the upper jaw is fused to the cartilaginous skull, and 
thus the hyoid does not help in its suspension (azéostylic) ; the skin is 
naked except in the young, which have some dorsal placoid spines. 
There is a urogenital aperture separate from the anus. In general 
the Holocephali most nearly resemble Plagiostomi, but they have many 
affinities with Dipnoi, ¢.g. in the autostylic skull. : 
Teeth (of Ptychodus, Rhynchodus, etc.), which have been referred to 
Chimeroids, occur in Devonian rocks, and some at least of the 
detached spines of Carboniferous age may have belonged to fishes 
of this order. Undoubted Mesozoic Chimeroids are Sgualoraja, 
Myriacanthus, Chimeropsis, Ischyodus, etc., while others, including 
the recent genus Chimera, are found in strata of Tertiary age. The 
other recent genus, Callorhynchus, is also represented by a Cretaceous 
species, C. hector?. 
EXTINCT ORDERS 
Order 3. PLEUROPTERYGII 
Devonian, Carboniferous, and Permian. Forms with unconstricted 
notochord, heterocercal tail, terminal mouth, paired fins with unseg- 
mented parallel radials. C/adoselache. 
Order 4. ICHTHYOTOMI 
Lower Carboniferous to Permian. Forms with unconstricted noto- 
chord, diphycercal tail, and pectoral fins with a segmented axis of 
basals bearing biserial radials. Plewracanthus, 
Order 5. ACANTHODEI 
Another interesting extinct group, whose position was for long a 
matter of dispute, but which is now usually placed near Elasmobranchii, 
is that of the Acanthodei. These flourished principally in Devonian 
times, but lived on through the Carboniferous to the Lower Permian. 
