THE SKULL 



77 



ment in a second group of these Fishes the Bony Ganoids 

 and gives rise to a dense armour composed of numerous bones 

 lying on the roof and extending into all parts of the skull and 

 jaws (Fig. 59j. The cartilage thus becomes reduced : it is, however, 

 largely retained in Amia. The opercular bones are more highly 

 developed than in cartilaginous Ganoids, and consist of an oper- 

 culum, a preoperculum, a suboperculum, and an interoperculum. 



If all the membrane bones are removed and the cranium separated from 

 the vertebral elements which are fused with it, a surprising similarity will be 

 seen between the skull of Polypterus and that of Elasmobranchs more par- 

 ticularly that of Chlamydoselache and Notidanus. On the other hand, the chon- 

 drocranium of Polypterus shows certain resemblances to that of the Amphibia. 



Fi. 58. CRANIAL SKELETON OF STURGEON (Acipenser) AFTER REMOVAL OF THE 



EXOSKELETAL PARTS. 



WS, vertebral column ; SpN, apertures for spinal nerves ; Psp, neural spines ; 

 Ob, neural arches ; C, notochord; GK, auditory capsule; PF, AF, postor- 



mandible ; De, dentary ; Ar, articular ; ffm, hyomandibular ; Sy, symplectic ; 

 Ih, interhyal ; hy, hyoid ; / to V, first to fifth branchial arches, with their 



segments the double pharyngo-branchial (a), the epibranchial (b) the cerato- 

 branchial (<), and the hypobranchial (d) ; Cop, basal elements of the visceral 

 skeleton ; Ri, ribs. 



The branchial skeleton in Ganoids consists of four or five more 

 or less strongly ossified gill-arches, decreasing in size antero- 

 posteriorly (Fig. 58) ; and in bony Ganoids the surface which looks 

 towards the throat is beset with teeth. 



The Ganoidei are of special interest, as they, with the Elasmobranchii, 

 constitute the entire Fish-fauna through the Silurian, Devonian, and Carbo- 

 niferous periods, and as the Teleostei which appear later, are doubtless derived 

 from them. They also show connection with the Dipnoi and with the oldest 

 Amphibia from the Carboniferous and Trias (Ganocephala, Stegocephala). 



In the Teleostei, the skull presents a large amount of varia- 

 tion ; its ground-plan, however, may always be derived from that 



