riSCES FISHES. 



History. The Elasmobranchs appear in the Upper Silurian, are 

 very abundant from the Carboniferous onwards, but are now greatly 

 out-numbered 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 Chiniicra from 

 northern seas, and Cattorhynckus from the south. There is a fold or 



FIG. 265. Lateral view of dog-fish (Scylli/tm catulus}. 



Note ventral mouth with naso-buccal groove, he % rocercal tail, and 

 unpaired fins, gs., Gill-slits; pc., pectoral fins; /?'., pelvic 

 fins. 



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 



FIG. 266. Outline of Acanthodes sulcatus. After Traquair. 

 /., Pectoral fins ; v., ventrals ; ., anal ; d., dorsal. 



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 Pfyctodtis, Rhynchodus, etc.), which have been referred to 



