568 PISCES FISHES. 



gills) and Heptane hus (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 Ckimara 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 (autostylic] ; 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, e.g. in the autostylic skull. 



Teeth (of Ptychodus, Rhynchodus> etc. ), which have been referred to 

 Chimseroids, 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 Chimseroids are Squaloraja, 

 Myriacanthus ', Chimaropsis, Ischyodus, etc., while others, including 

 the recent genus Chimcera^ are found in strata of Tertiary age. The 

 other recent genus, Callorhynchus^ is also represented by a Cretaceous 

 species, C. htctori. 



EXTINCT ORDERS 



Order 3. PLEUROPTERYGII 



Devonian, Carboniferous, and Permian. Forms with unconstricted 

 notochord, heterocercal tail, terminal mouth, paired fins with unseg- 

 mented parallel radials. Cladoselache. 



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. Pleuracanthus. 



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. 



