66 



( < iMPARATIVK AN AT( )MY. 



Amongst the Ganoids, tin- lowest condition is met with in 

 those forms in \vhirh the hyaline primordial skull, immovably 

 fixed to the vertebral column, is still retained. These forms are 

 spoken of as Cartilaginous Ganoids. The appearance of 

 definite bones, however, divides them sharply off from the Elas- 

 m.>l,ranchs, and pn.ycs tlu-ir skull 1.. be at a much high-T stage of 

 development. These bones ha\e the form of richly sculptured 

 plates and shields, and are developed partly from the mucous 

 membrane >f the mouth (as f..r instance the parasphenoid, which 

 lies alone; the base of the skull), and partly from the outer skin 

 (compare the ehapter on the dermal skeleton, p. 3'1). 



7? 



Me/ 



Fi<:. 52. CRANIAL SKELETON OF STUIIC;EO\ (.-1,-ipenser) AFTER REMOVAL OF THE 



EX( 'SKELETAL I'.MtTS. 



//',s', vertebral cohinni : ,S'/^V, apertures for spinal nerves : /'/<, spinous processes ; 

 /i, neural arches ; C, notocliord ; UK, auditory capsule : /'/', AF, postorbital 

 mill ;inti.rl)it.;il processes; Orb, orbit ; Jf, optic foramen ; ./, A'agus t'oranicn ; 

 Ait, nasiil cavity; li, rostrum; *, prominent riil^e on the kisis .-ranii ; /\-, /'.s j , 

 7V, paraspheimiil ; PQ, palatoqnadrute ; >.jn, quadrate; Mil, mandible; DC, 

 dentary; .//, articular; /////, hyomandibular ; A'//, symplectic ; ///, intcrhyal ; 

 /"/, liy'iiid ; / to /', first to fifth branchial arches, with their segments the 



jiharyn^o-branehial (ft}, tin- epilii-ancliial (/<), the cerato-branchial (<), and 

 r.incliial ('/) ; <.'op, basal elements of the visceral skeleton ; Hi, ribs. 



the liypii 



This dermal skeleton attains to a much more considerable 

 development in a second group of these Fishes the bony 

 ( !;moids and gives rise to a strong armour composed of numerous 

 pieces lying on the roof of the skull (Fig. 53). The ossifications are 

 not restricted to the outer surface, but extend into all parts of the 

 skull, as, for instance, the trabecular regions and the lower jaw ; 

 the cartilage thus becomes -reatly reduced. 



The branchial skeleton in Ganoids consists of four or five mon- 

 or le.-s strongly ossified gill-arches, decreasing in size antero- 

 p isteriorly. In bony Ganoids the surface which looks towards the 

 throat is beset with teeth. A gill-cover, often supported by several 

 bony pieces, is alwa\s present. 



In the form of their skull, the Dipnoi show many points of con- 

 nection with Elasmobranchs, Ganoids, Tel osteans, and Urodeles. 



