THE CRANIAL SKELETON. 



21 



an obtuse angle with the hitter. In the former case, the ex- 

 tremity of the spine divides the caudal fin-rays into two nearly 

 equal moieties, an upper and a lower, and the fish is said to 

 be diphy cereal (Fig. 6, A). In the latter case, the upper di- 

 vision of the caudal fin-rays is much smaller than the lower, 

 and the fish is heterocercal (Fig. 6, B, C). 



In most osseous fishes the hypural bones which support 

 the fin-rays of the inferior division become much expanded, 

 and either remain separate, or coalesce into a wedge-shaped, 

 nearly symmetrical bone, which becomes anchylosed with the 

 last ossified vertebral centrum. The inferior fin-rays are now 

 disposed in such a manner as to give the tail an appearance 

 of symmetry with respect to the axis of the body, and such 

 fishes have been called homocercal. Of these homocercalfish, 

 some (as the Salmon, Fig. 6) have the notochord unossified, 

 and protected only by bony plates developed at its sides. In 

 others (as the Stickleback, Percn, etc.), the sheath of the no- 

 tochord becomes completely ossified and united with the cen- 

 trum of the last vertebra, which then appears to be prolonged 

 into a bony urostyle. 



Fig, 7. — ^The cartilaginous cranium of a Fowl at the sixth d^y of incubation, riewed from be- 

 low. — P, the pituitary space ; tr, the tralieculce^ uniting in front, in the bifurcated eth- 

 moYomerine plate; Qu. the quadrate cartilage; /Sb, the semicircular canals; C'o, the 

 cochlea ; A, the notochord imbedded in the basilar plate. 



TJie Cranial System. — As has been stated, no protover- 

 tebrse appear on the floor of the skull ; nor is there any era™ 

 nium, nor any developmental stage of a cranium, in which sep- 

 arate cartilaginous centres are known to occur in this region. 



On the contrary, when chondrification takes place, it ex- 

 tends continuously forward, on each side of the notochord, 



