Xll 
rNTRODIJCTION’. 
higher groups, and must have been contemporaneous with many 
unknown Acanthopterygii which rapidly became differentiated into 
various families in seas of which the sediments still remain 
undiscovered or unexplored. Palaeontology has, indeed, hitherto 
revealed as little concerning the origin of the dominant Tertiary 
fishes as of the Tertiary mammals. According to present knowledge, 
all the principal types had already appeared early in the Eocene 
period; and among these fishes there are many which cannot be 
distinguished by their skeleton from genera which still survive. 
The Eocene Acanthopterygii are best known by imperfect skulls 
and other uncrushed fossils from the Lower Eocene London Clay of 
Sheppey, and by compressed skeletons from the fissile Upper Eocene 
limestone of ISTorthern Italy. Freshwater fishes, perhaps of the 
same age, are also represented by good skeletons in the Green Liver 
Shales of Wyoming, U.S.A. Among the Berycoids there are none 
particularly remarkable; but the Scombroid family of Carangidae 
includes some of its most specialised and aberrant genera, such as 
Vomeropsis and Mene^ of which the latter has survived unchanged 
until the present day. Among extinct fishes to be placed near the 
Scombroids, the most remarkable are the highly specialised Palseo- 
rhynchidse, which range from the Lower Eocene to the Lower 
Miocene. Among Percoids the fragmentary fossils suggest that the 
families were less differentiated in the Eocene period than they are 
at the present day ; but one curious specialisation, the fusion of the 
lower pharyngeal bones now so characteristic of the Labridse and 
Chromidse, had already been acquired by several genera. The 
Plectognathi were also less completely differentiated from the 
Chaetodonts than they are in the existing fauna. The Scorpaenoids 
are represented in the Lower Eocene by one imperfectly known 
genus (^Amplieristus), which has a typically armoured skull and 
cheek. The Cottoids, Gobioids, and Blennioids also appear to be 
recognisable; and with the latter may possibly be associated the 
problematical genus Blocliius^ which does not range beyond Eocene 
formations. 
Even after the Eocene period the history of the Acanthopterygii 
cannot be clearly traced. Most of the known fossils are too 
imperfect for satisfactory determination ; for the constant distinctive 
characters of these highly specialised fishes are of a minor kind and 
often quite inconspicuous compared with those which permit the 
classification of the lower grades. Moreover, a very large pro¬ 
portion of the species are represented solely by the detached otoliths, 
