=The Square-tails: Tetragonuridæ.=—The _Tetragonuridæ_ are long-bodied fishes of a plump or almost squarish form, covered with hard, firm, very adherent scales. _Tetragonurus cuvieri_, the single species, called square-tail, or escolar de natura, is a curious fish, looking as if whittled out of wood, covered with a compact armor of bony scales, and swimming very slowly in deep water. It is known from the open Atlantic and Mediterranean and has been once taken at Wood's Hole in Massachusetts. According to Mr. C. T. Regan the relations of this eccentric fish are with the _Stromateidæ_ and _Bramidæ_, the skeleton being essentially that of _Stromateus_, and Boulenger places both _Tetragonurus_ and _Stromateus_ among the _Percesoces_. =The Crested Bandfishes: Lophotidæ.=—The family of _Lophotidæ_ consists of a few species of deep-sea fishes, band-shaped, naked, with the dorsal of flexible spines beginning as a high crest on the elevated occiput. The first spine is very strong. The ventrals are thoracic with the normal number, I, 5, of fin-rays. _Lophotes cepedianus_, the crested bandfish, is occasionally taken in the Mediterranean in rather deep water. _Lophotes capellei_ is rarely taken in the deep waters of Japan. It is thought that the _Lophotidæ_ may be related to the ribbon-fishes, _Tæniosomi_, but on the whole they seem nearer to the highly modified _Scombroidei_, the _Pteraclidæ_ for example. In a natural arrangement, we should turn from the _Bramidæ_ to the _Antigoniidæ_ and the _Ilarchidæ_, then passing over the series which leads through _Chætodontidæ_ and _Teuthidæ_ to the _Plectognaths_. It is, however, necessary to include here, alongside the mackerels, though not closely related to them, the parallel series of perch-like fishes, which at the end become also hopelessly entangled, through aberrant forms, with other series of which the origin and relations are imperfectly understood. As the relations of forms cannot be expressed in a linear series, many pages must intervene before we can take up the supposed line of development from the Scombroid fishes to those called _Squamipinnes_. CHAPTER XVIII PERCOIDEA, OR PERCH-LIKE FISHES =PERCOID Fishes.=—We may now take up the long series of the _Percoidea_, the fishes built on the type of the perch or bass. This is a group of fishes of diverse habits and forms, but on the whole representing better than any other the typical _Acanthopterygian_ fish. The group is incapable of concise definition, or, in general, of any definition at all; still, most of its members are definitely related to each other and bear in one way or another a resemblance to the typical form, the perch, or more strictly to its marine relatives, the sea-bass, or _Serranidæ_. The following analysis gives most of the common characters of the group: