These types, represented by the mullet, the silverside, and the barracuda, have been segregated by Cope as an order called _Percesoces_ (Perca, perch; Esox, pike), a name which correctly describes their real affinities. In these typical forms, mullet, silverside, and barracuda, the affinities are plain, but in other transitional forms, as the threadfin and the stickleback, the relationships are less clear. Cope adds to the series of _Percesoces_ the _Ophiocephalidæ_, which Gill leaves with the _Anabantidæ_ among the spiny-rayed forms. Boulenger adds also the sand-lances (_Ammodytidæ_) and the threadfins (_Polynemidæ_), while Woodward places here the _Crossognathidæ_. In the present work we define the _Percesoces_ so as to include all spiny-rayed fishes in which the ventral fins are naturally abdominal, excepting those having a reduced number of gill-bones, or of actinosts, or other peculiarities of the shoulder-girdle. The _Ammodytidæ_ have no real affinities with the _Percesoces_. The _Crossognathidæ_ and other families with abdominal ventrals and the dorsal spines wholly obsolete may belong with the _Haplomi_. Boulenger places the _Chiasmodontidæ_, the _Stromateidæ_, and the _Tetragonuridæ_ among the _Percesoces_, an arrangement of very doubtful validity. In most of the _Percesoces_ the scales are cycloid, the spinous dorsal forms a short separate fin, and in all the air-duct is wanting. =The Silversides: Atherinidæ.=—The most primitive of living _Percesoces_ constitute the large family of silversides (_Atherinidæ_), known as "fishes of the King," Pescados del Rey, Pesce Rey, or Peixe Re, wherever the Spanish or Portuguese languages are spoken. The species are, in general, small and slender fishes of dry and delicate flesh, feeding on small animals. The mouth is small, with feeble teeth. There is no lateral line, the color is translucent green, with usually a broad lateral band of silver. Sometimes this is wanting, and sometimes it is replaced by burnished black. Some of the species live in lakes or rivers, others in bays or arms of the sea, but never at a distance from the shore or in water of more than a few feet in depth. The larger species are much valued as food, the smaller ones, equally delicate, are fried in numbers as "whitebait," but the bones are firmer and more troublesome than in the smelts and young herring. The species of the genus _Atherina_, known as "friars," or "brit," are chiefly European, although some occur in almost all warm or temperate seas. These are small fishes, with the mouth relatively large and oblique and the scales rather large and firm. _Atherina hepsetus_ and _A. presbyter_ are common in Europe, _Atherina stipes_ in the West Indies, _Atherina bleekeri_ in Japan, and _Atherina insularum_ and _A. lacunosa_ in Polynesia. The genus _Chirostoma_ contains larger species, with projecting lower jaw, abounding in the lakes of Mexico. _Chirostoma humboldtianum_ is very abundant about Mexico City.