THE SKELETON IN FISHES. THE SKULL. 117 



to two-bladed tails in which the spinal column forms a definite 

 axis running through the dorsal blade, while the ventral blade 

 is enlarged and generally forms the functional part of the 

 tail. Such heterocercal tails are found in nearly all Elasmo- 

 branchii, together with the living cartilaginous Ganoidei, 

 and many extinct forms belonging to the same order; Lepi- 

 dosteus, Amia, and the Dipteridae among Dipnoi, have tails 

 which, though obviously heterocercal, are not two-bladed. 



The vast majority of the Teleostei and some extinct Ga- 

 noidei have heterocercal tails of the modified type to which 

 the term homocercal is applied. The hypural bones which 

 support the lower half of the tail fin become much enlarged, 

 and frequently unite to form a wedge-shaped bone which be- 

 comes ankylosed to the last ossified vertebral centrum. The 

 fin-rays then become arranged in such a way as to produce a 

 secondary appearance of symmetry. Some homocercal fish 

 such as the Perch have the end of the notochord protected 

 by a calcified or completely ossified sheath, the urostyle, to 

 which several neural and haemal arches may be attached, and 

 which becomes united with the centrum of the last vertebra ; 

 in others such as the Salmon the end of the notochord is pro- 

 tected only by laterally placed bony plates. 



THE SKULL. 



It is often impossible to draw a hard and fast line between 

 the cranium and the vertebral column. This is the case for 

 instance in Acipenser (fig. 18, 16) among Chondrostei, in Amia 

 among Holostei, and in Ceratodus and Protopterus among 

 Dipnoi. The occipital region of the skull in Amia is clearly 

 formed of three cervical vertebrae whose centra have become 

 absorbed into the cranium, while the neural arches and spines 

 are still distinguishable. 



The simplest type of cranium is that found in ELASMO- 

 BRANCHS: it consists of a simple cartilaginous box, which is 

 generally immovably fixed to the vertebral column, though 



