CERATODUS 81 



central axis bearing radials both in front and behind, a con- 

 dition known as biserial, and they conform to the type known 

 as " archipterygial." Both the paired fins and the dorsal 

 fin are covered with scales. 



The gill-slits, five in number, are protected by an oper- 

 culum. The spiracle is closed. The anterior nostrils are on 

 the ventral surface of the snout, and behind them are the 

 posterior nostrils which open into the mouth. There is a 

 cloaca into which the alimentary, excretory, and genital systems 

 open, as also do a pair of abdominal pores. 



Scales. — The scales are thin and covered with spines which 

 must not be mistaken for denticles. They overlap one another 

 from before backwards as in Gadus, and they also extend over 

 the dermal bones of the skull and the paired fins, and the 

 dorsal fin. 



Fin-rays. — The dermal fin-rays are jointed and made of 

 fibrous substance. They differ from the lepidotrichia of 

 Gadus in being more numerous than the radials, and in being 

 covered over by scales. No actinotrichia are present, and it 

 is uncertain whether these rays, which are called camptotrichia, 

 represent the ceratotrichia or the lepidotrichia of Gadus. 



Skull. — The skeleton is largely cartilaginous, and little 

 of this primitive skeleton is replaced by bone. The neuro- 

 cranium forms a complete case enclosing the brain, olfactory 

 and auditory capsules, and several vertebrae are plastered on 

 to its hind end. As some of these bear ribs, the latter appear 

 to articulate with the skull, and are called cranial ribs. The 

 only cartilage-bone in the neurocranium represents one of the 

 neural arches which have been incorporated as just described. 

 The membrane-bones which cover the dorsal surface of the 

 neurocranium are sunk beneath the surface of the skin and are 

 themselves overlain by scales. These bones are very modified 

 and secondary, there is a preponderance of median unpaired 

 bones, and as they cannot well be compared with those of 

 other forms, there is little advantage in studying them in 

 detail. On the underside of the neurocranium are to be found 

 a parasphenoid, prevomer, and paired ptery go-palatines. 



The splanchnocranium is important because of the manner 



G 



