CHAPTER XVI 
PERCOMORPHI 
UBORDER Percomorphi, the Mackerels and Perches. — 
We may place in a single suborder the various groups 
of fishes which cluster about the perches, and the 
mackerels. The group is not easily definable and may con- 
tain heterogeneous elements. We may, however, arrange 
in it, for our present purposes, those spiny-rayed fishes 
having the ventral fins thoracic, of one spine and five rays 
(the ventral fin occasionally wanting or defective, having a 
reduced number of rays), the lower pharyngeal bones separate, 
the suborbital chain without backward extension or bony 
stay, the post-temporal normally developed and separate from 
the cranium, the premaxillary and maxillary distinct, the 
cranium itself without orbitosphenoid bone, having a structure 
not greatly unlike that of perch or mackerel, and the back- 
bone primitively of twenty-four vertebra, the number increased 
in arctic, pelagic, or fresh-water offshoots. ; 
The species, comprising the great body of the spiny-rayed 
forms, group themselves chiefly about two central families, 
the Scombride, or mackerels, and the Serranide, the sea-bass, 
with their fresh-water allies, the Percide, or perch. 
The Mackerel Tribe: Scombroidea——The two groups of Per- 
comorphi, the mackerel-like and the perch-like, admit of no 
exact definition, as the one fully grades into the other. The 
mackerel-like forms, or Scombroidea, as a whole are defined by 
their adaptation for swift movement. The profile is sharp an- 
teriorly, the tail slender, with widely forked caudal; the scales 
are usually small, thin, and smooth, of such a character as not 
to produce friction in the water. 
In general the external surface is smooth, the skeleton 
light and strong, the muscles firm, and the species are carniv- 
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