SKULL 



89 



divides them sharply off from Elasmobranchs, and shows that 

 their skull has reached a much higher stage of development. 

 These bones have the form of richly sculptured plates and shields, 

 and are developed partly from the mucous membrane lining the 

 mouth and covering the visceral skeleton, and partly from the skin 

 covering the roof of the skull, where the arrangement of the bones 

 (e.g. frontals and parietals) typical of higher forms can to some 

 extent be recognised. A narrow parasphenoid forms a roof to the 

 oral cavity. The operculurn is more pronounced than in the 

 Holocephali, and is also supported by bones (cf. p. DO). The 

 whole palato-mandibular apparatus which is comparatively small, 



M(g 



FIG. 67. CRANIAL SKELETON OK STURGEON (Acipemer) AFTER REMOVAL OF THE 



EXOSKELETAI. PARTS. 



Ar, articular; C, notochorcl ; Cop, basal elements of the visceral skeleton ; De, 

 dentary ; GK, auditory capsule ; Hm, hyomandibular ; hy, hyoid ; 7 to V, 

 first to fifth branchial arches, with their segments the double pharyngo- 

 branchial (a), the epibranchial (b), the ceratobranchial (c), and the hypo- 

 branchial (d) ; Ih, interhyal; //, optic foramen ; Md, mandible ; Na, nasal 

 cavity ; Ob, neural arches ; Orb, orbit ; PF, AF, postorbital and antorbital 

 processes; PQ, palatoquadrate ; P8,'Ps', Ps", parasphenoid; Psp, neural 

 spines ; Qu, quadrate ; JR, rostrum ; Ri, ribs ; /S?jJV, apertures for spinal 

 nerves ; Sy, symplectic ; WS. vertebral column ; x, vagus foramen ; *, pro- 

 minent ridge on the basis cranii. 



bears no teeth, and in relation with which bones are formed is 

 connected very loosely with the skull by means of a hyoman- 

 dibular and symplectic, as well as by ligaments (Fig. 67). 



The dermal skeleton attains a much more considerable develop- 

 ment in the Bony Ganoids (Crossopterygii and Holostei), 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. 68, A and B) : amongst these may be noted a median 

 (Amia) or paired (Polypterus) jugular plate between the rami of 

 the mandible. In addition to the investing bones, replacing bones 

 are present in the occipital, obic, orbitotemporal, and ethmoidal 

 regions. Investing and replacing bones very similar to those 



