Fig. 178.—Shoulder-girdle of a Threadfin, Polydactylus approximans (Lay & Bennett).
Fig. 179.—Threadfin, Polydactylus octonemus (Girard). Pensacola.
The pectoral fin is divided into two parts, the lower composed of free or separate rays very slender and thread-like, sometimes longer than the body. Two of the actinosts of the pectoral support the fin, one is slender and has no rays, while the fourth is plate-like and attached to the coracoids, supporting the pectoral filaments. The body is rather robust, covered with large scales, formed much as in the mullet. The lateral line extends on the caudal fin as in the Sciænidæ which group these fishes resemble in many ways. The mouth is large, inferior, with small teeth. The species are carnivorous fishes of excellent flesh, abounding on sandy shores in the warm seas. They are not very active and not at all voracious. The coloration is bluish and silvery, sometimes striped with black. Most of the species belong to the genus Polydactylus. Polydactylus virginicus, the barbudo, with seven filaments, is common in the West Indies and Florida. Polydactylus octonemus with eight filaments is more rare, but ranges further north. Polydactylus approximans, the raton of western Mexico, with six filaments, reaches San Diego. Polydactylus plebejus is common in Japan and other species range through Polynesia. In India isinglass is made from the large air-bladder of species of Polydactylus. The rare Polynemus quinquarius of the West Indies have five pectoral filaments, these being greatly elongate, much longer than the body.
No extinct Polynemidæ are recorded.
CHAPTER XIII
PHTHINOBRANCHII: HEMIBRANCHII, LOPHOBRANCHII,
AND HYPOSTOMIDES
Fig. 180.—Shoulder-girdle of a Stickleback, Gasterosteus aculeatus Linnæus. (After Parker.)
Fig. 181.—Shoulder-girdle of Fistularia petimba Lacépède, showing greatly extended interclavicle, the surface ossified.