498 AETHEOSPONDYLI [CH. 



to the myotome in front. Vertebrae which have hollow articular 

 surfaces behind are termed opisthocoelous (Gr. OTTIO-^O-, behind). 



The skull is just as well ossified as that of a Teleostean, but it 

 lacks the supra-occipital, the cartilage in that region being covered 

 here by two dermal bones called supratemporals. 



The upper jaw articulates with the pro-otic behind the eye, so 

 that the jaw is doubly supported, since hyomandibular and sym- 

 plectic are also developed, but its amphistylism is different from 

 that of Teleostei. Covering the inner side of the lower jaw mem- 

 brane bones, termed splenial and supra -angular are developed. 



The tail fin is a rounded lobe derived from the ventral lobe of 

 a heterocercal tail. The vestigial dorsal lobe is perhaps represented 

 by a vertical line of fulcra (i.e. /^-shaped ganoid scales), which 

 cover its anterior border. 



The jaws are long and bill-like, and the maxilla is represented 

 by several bones in series. The fish lies in wait for its prey amidst 

 reeds on the shallow border of lakes. Its air-bladder is produced 

 into pockets like the alveoli of the lung of a land animal, and the 

 fish swallows air and emits it, so that it is probable that the air- 

 bladder is partly used as a respiratory organ. 



Order III. Protospondyli. 



The Protospondyli include, like the Aetheospondyli, only a 

 single genus, Amia, represented by a single species, Amia calva, 

 the Bow-fin, inhabiting the lakes and rivers of North America. The 

 Protospondyli show the same archaic features as the preceding 

 group in the matter of vasa efferentia, conus, optic chiasma and 

 spiral valve ; further, they have short wide oviducts opening 

 internally into the body cavity by wide funnels, whereas in 

 Aetheospondyli, as in most Teleostei, ovary and oviduct are con- 

 tinuous. The Protospondyli are, above all, distinguished by the 

 structure of the vertebral column. In the young fish dorsal and ven- 

 tral intercalaries are present as well as neural and haemal arches, 

 and all these arch pieces become connected with each other by 

 bony rings which form centra, but the centra so formed resemble 

 those of Teleostei in being amphicoelous and enclosing between 

 them portions of the notochord, and in the tail the centra are 

 doubly as numerous as the myotomes, a centrum bearing dorsal 

 and haemal arches alternating with one devoid of both. From 

 the study of development it appears that the arch-bearing 

 centrum is formed chiefly by a bony ring connecting the basi- 



