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Threadfishes
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THIS is a small family of tropical fishes bearing superficial resemblances to the Mugilidae on the one hand and the Scioenidae on the other. They may be distinguished by the abdominal ventrals, the presence of 2 dorsal fins, the anterior of spines only, and by having the lower 5 to 8 rays of the ventral fin detached and filamentous. The family contains 4 genera and about 25 species, only 2 of the former and about 5 of the latter being found in our waters.
The 2 genera of this family (Polynemus and Polydactylus) both have species in our waters, only those of the latter being of any value.
Polydactylus differs from Polynemus in the shorter soft dorsal, the serrate preopercle, and the toothed vomer. Numerous species in warm seas, but only 4 within our limits.
a. Pectoral filaments 6, rarely 5; ..................approximans
aa. Pectoral filaments 7; ..................................virginicus
b. Maxillary less than 1/2 length of head; pectoral filaments 8; octonemus
bb. Maxillary more than 1/2 length of head; pectoral filaments usually 9; ........................opercularis
Polydactylus approximans. whose only vernacular name is raton, is found on the Pacific Coast of tropical America from San Diego to Panama. It reaches a foot in length and is a common food-fish at Guaymas, Mazatlan and Panama.