SKELETON. 



S33 



Tmx 



vertebral column has developed, is finally represented only 

 by vestiges in the centra of the vertebrae. 



The skull consists (a) of the persistent parts of the 

 original cartilaginous brain box or chondrocranium, de- 

 veloped, as in the skate, from parachordals and trabecute, 



plus nasal and auditory 

 capsules ; {b) of ossifi- 

 cations of parts of the 

 chondrocranium, cartilage 

 bones ; (c) of membrane 

 or investing bones ; and 

 (d) of associated visceral 

 arches. 



Part of the chondrocranium 

 persists as an encasement of 

 the brain. Two exoccipitals 

 bounding the foramen magnum 

 and forming the condyles, two 

 pro-otics or ossifications of the 

 original auditory capsule, and 

 an unpaired sphenethmoid 

 forming the front of the brain- 

 case are cartilage bones. Two 

 parieto-frontals and two nasals 

 above, a paired vomer and an 

 unpaired dagger shaped para- 

 sphenoid beneath, and two 

 lateral hammer shaped squa- 

 mosals are membrane bones. 

 There is no basisphenoid ossi- 

 fication. 



To these are added the small 

 pre-maxilloa in the very front 

 of the skull, the long'maxillie 

 on each side, the cjuadrato- 

 jugals which continue the 

 latter to the minute nodule 

 which represents the quadrate 

 bone. 



On the roof of the mouth, 

 extending from the quadrate 

 forwards to near the vomers, 

 are the triradiate pterygoids, 

 while at right angles to the 

 parasphenoid and behind the vomers are the 



Fig. 176.— Skull of Frog— upper 

 and lower surface. (After W. K. 

 Parker.) 



Upper surface — 



Pmx^ premax-illa ; N, nasal ; M, max- 

 illa ; Sg, squamosal; Qj\ quadrato-jugal ; 

 e.o., ex-occipitals ; P./., parieto-frontals : 

 Spk,E, sphenethmoid ; P.O,^ pro-otic. 



Lower surface — 



Pmx, premaxilla ; Af, maxilla ; Qi, quad- 

 rato-jugal : Q, quadrate ; Pt, pterygoid ; 

 Ps, parasphenoid; P.O., pro-otic; .Sp.E, 

 sphenethmoid ; PI, palatine ; V, vomer ; c, 

 columella. 



the 



anterior end of 

 palatines. 



Each half of the lower jaw, based on Meckel's cartilage, consists of 



