FIG. 177.—_Cobitopsis acuta_ Gervais, restored. Oligocene of Puy-de-Dôme. (After Woodward.) ] =Suborder Rhegnopteri.=—The threadfins (_Polynemidæ_) are allied to the mullets, but differ from them and from all other fishes in the structure of the pectoral fin and its basal bones, or actinosts. [Illustration: FIG. 178.—Shoulder-girdle of a Threadfin, _Polydactylus approximans_ (Lay & Bennett). ] [Illustration: 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 [Illustration: FIG. 180.—Shoulder-girdle of a Stickleback, _Gasterosteus aculeatus_ Linnæus. (After Parker.) ] [Illustration: FIG. 181.—Shoulder-girdle of _Fistularia petimba_ Lacépède, showing greatly extended interclavicle, the surface ossified. ]