GANOIDEI. 507 



The Holocephali are represented by the sea cat or Chiniivra from 

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

 operculum covering the gill clefts and leaving only one external opening 

 on each side ; the jaws are rigidly fixed to the cartilaginous skull ; the 

 skin is naked ; the anus, the IMiillerian and urinary ducts open sepa- 

 rately. Otherwise the Holocephali resemble Elasmobranchs, and may 

 be regarded as a suborder. In some respects, however, e.g. , in the 

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

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



Teeth {oi Ptyctodics^ Rhyiichodiis^ &c. ), which have been referred to 

 Chimasroids 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 Chimceroids are Sqiialoraja, Myria- 

 canthus^ Chiniitropsis^ Ischyodus, &c. , while others, including the recent 

 genus Chi77iara, are found in strata of Tertiary age. The other recent 

 genus, Callorhynchus ^ is also represented by a Cretaceous species, 

 C. Hectori. 



Another interesting but quite extinct group, whose position was for 

 long a matter of dispute, but which is now usually referred to the 



Fig. 171. — Outline of Acanthodes subcatus. (After 

 Traquair.) 



/., Pectoral fins ; 7'., ventrals ; «., anal ; d., dorsal. 



Elasmobranchii, is that of the Acanthodei. These flourished principally 

 in Devonian times, but lived on also through the Carboniferous to the 

 Lower Permian. These are usually rather small fishes, with minute 

 rhomboidal shagreen-like scales, and a strong spine in front of each 

 fin, except the caudal. In some genera (Parexus, Clitnatius) there are 

 two rows of small intermediate spines between the proper pectorals and 

 the ventrals. 



Order II. Ganoidei. 



This ancient order of armoured fishes flourished in 

 Devonian and Carboniferous ages, but is now represented 

 by only seven genera, of which the Sturgeon {Acipenser) and 

 the Bony Pike {Lepidosteus) are the most famihar. 



The skin bears large scales, or bony scutes. The tail is 

 either heterocercal or homocercal. Membrane bones invest 



