HOLOCEPHALI. 



5'7 



body, a continuous dorsal fin, a symmetrical tail, and pectoral fins,'with an 

 arrangement of rays resembling that in the biserial "archipterygium." 



Holocephali. 



The Holocephali are represented by the sea-cat or Chimara from 

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

 operculum covering the gill-clefts and leaving only one external opening 



Fig. 220. — Lateral view of dog-fish [Scyllium catulus). 



Note ventral mouth with naso-buccal groove, heterocercal tail, and 

 unpaired fins, gs.. Gill-slits; pc, pectoral fins; pv., pelvic 

 fins. 



on each side ; the upper jaw is fused to the cartilaginous skull ; the 

 skin is naked ; the anus, the Mullerian and urinary ducts, open 

 separately. Otherwise the Holocephali resemble Elasmobranchs, and 

 may be regarded as a sub-order. In some respects, however, e.g. in the 

 structure of the skull, they suggest Dipnoi, and in the connection it is 

 interesting to notice that there is an auricular septum in Chimara. 



Fig. 221. — Outline ot Acanthodes suicatus. — After Traquair. 

 p., Pectoral fins ; v., ventrals ; a., anal ; d., dorsal. 



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

 Chirmeroids, occur in Devonian rocks, and some at least of the detached 

 spines of Carboniferous age may have belonged to fishes of this order 

 or sub-order. Undoubted Mesozoic Chimseroids are Squaloraja, 

 Myriacanthus, Chimceropsis, Ischyodus, etc. , while others, including the 

 recent genus Chimtzra, are found in strata of Tertiary age. The 

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

 species, C. Hectori. 



