XVII HOLOSTEI AG7 
The Fishes here included in the Holostei constitute the 
Protospondyli and Aetheospondyli of Smith Woodward.’ 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 two 
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 familes which 
at the present day are the sole survivors of the group. 
Of the six families of Protospondyl the Semionotidae are the 
oldest and most generalised, and the Macrosemiidae a closely 
allied group. The Pyenodontidae 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. 
Fic. 292.—Restoration of Lepidotus minor. Upper Jurassic, Dorset. x 4. 
(After Smith Woodward.) 
Fam. 1. Semionotidae.—Small-mouthed, fusiform or deep- 
1 Brit. Mus. Cat. Foss, Fishes, iii. pp, 48, 415. 
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