CHAPTER XXVI 
ACANTHOPTERYGII; SYNENTOGNATHI 
SiIRDER <Acanthopterygii, the Spiny-rayed Fishes. — The 
most of the remaining bony fishes constitute a natural 
fe— AF) group for which the name Acanthopterygi (axavia, 
spine; zréov&, zrepor, fin or wing) may be used. This name 
is often written Actinopteri, a form equally correct and more 
. euphonious and convenient. These fishes are characterized, 
with numerous exceptions, by the presence of fin spines, by 
the connection of the ventral fins with the shoulder-girdle, 
by the presence in general of more than one spine in the an- 
terior part of dorsal and anal fins, and as a rule of one spine and 
five rays in the ventral fins, and by the absence in the adult 
of a duct to the air-bladder. Minor characters are these: 
the pectoral fins are inserted high on the shoulder-girdle, the 
scales are often ctenoid, and the edge of the upper jaw is formed 
by the premaxillary alone, the maxillary being always toothless. 
But it is impossible to define or limit the group by any 
single character or group of characters. It is connected with 
the Malacopterygi through the Haplomi on the one hand by 
transitional groups of genera which may lack any one of these 
characters. On the other hand, in the extreme forms, each 
of these distinctive characters may be lost through degenera- 
tion. Thus fin spines, ctenoid scales, and the homocercal tail 
are lost in the codfishes, the connection of ventrals with shoulder- 
girdle fails in the Percesoces, etc., and the development of the 
air-duct is subject to all sorts of variations. In one family 
even the adipose fin remains through all the changes and 
modifications the species have undergone. 
The various transitional forms between the Haplom: and 
the perch-like fishes have been from time to time regarded as 
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