xvni HOLOSTEI 497 



The Fishes here included in the Holostei constitute the 

 Protospondyli and Aetheospondyli of Smith Woodward. 1 In the 

 former group vertebral centra are either entirely absent, or, if 

 present, their components in the form of alternating hypo- and 

 pleuro-centra invariably remain distinct in the tail. The latter 

 group has been instituted for the provisional reception of t\vo 

 highly specialised families of uncertain relationships, which differ 

 from the Protospondyli in their higher grade of vertebral 

 structure, the centra always being complete without any indica- 

 tion of distinct hypo- and pleuro-centra. 



The Holostei first appear in the Permian, where they are 

 represented by a single genus (Acentrophorus). During the 

 Mesozoic period they were abundant in the Trias, reaching their 

 maximum development and becoming the dominant Fishes of the 

 period in the Jurassic. In the Cretaceous they began to decline, 

 and in the Tertiaries became reduced to the two families which 

 at the present day are the sole survivors of the group. 



Of the six families of Protospondyli the Semionotidae are the 

 oldest and most generalised, and the Macrosemiidae a closely 

 allied group. The Pycnodontidae are a highly specialised and 

 terminal offshoot. The Eugnathidae obviously lead to the 

 Amiidae, and from the same stock it is probable that the Pachy- 

 cormidae have been derived. The relations of the Aspidorhyn- 

 chidae and Lepidosteidae (Aetheospondyli) are extremely doubtful. 

 That the two families are allied seems probable, but beyond the 

 possibility of a remote connection with the Protospondyli there is 

 no clue to their ancestry. 



FIG. 292. Restoration of Lepidotus minor. Upper Jurassic, Dorset, x . 

 (After Smith Woodward.) 



Fam. 1. Semionotidae. Small -mouthed, fusiform or deep- 



1 Brit. Mus. Cat. Foss. Fishes, iii. pp. 48, 415. 

 VOL. VII 2 K 



