CHAPTER XXIX 
SALMOPERCAZ AND OTHER TRANSITIONAL 
GROUPS 
UBORDER Salmoperce, the Trout-perches: Percopside. 
—More ancient than the Hemibranchit, and still more 
distinctly in the line of transition from soft-rayed to 
spiny-rayed fishes, is the small suborder of Salmoperce. This 
is characterized by the presence of the adipose fin of the salmon, 
Fic. 360.—Sand-roller, Pecropsis guttatus Agassiz. Okoboji Lake, Ia. 
in connection with the mouth, scales, and fin-spines of a perch. 
The premaxillary forms the entire edge of the upper jaw, the 
maxillary being without teeth. The air-bladder retains a 
rudimentary duct. The bones of the head are full of mucous 
cavities, as in the European perch called Gymnocephalus and 
Acerina. There are two spines in the dorsal and one or two 
in the anal, while the abdominal ventrals have each a spine and 
eight rays. Two species only are known among living fishes, 
these emphasizing more perfectly than any other known forms 
the close relation really existing between spinous and _soft- 
rayed forms. The single family of Percopside would seem to 
find its place in Cretaceous rocks rather than in the waters of 
to-day. 
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