The Plectognaths.—Derived directly from the Acanthuridæ, from which they differ by progressive steps of degeneration, are the three suborders of Sclerodermi, Ostracodermi, and Gymnodontes, forming together the series or suborder of Plectognathi. As the members of this group differ from one another more widely than the highest or most generalized forms differ from the Acanthuridæ, we do not regard it as a distinct order. The forms included in it differ from the Acanthuridæ much as the swordfishes differ from ordinary mackerel. The Plectognathi (πλεϡτός, woven together; γνάθος, jaw) agree in the union of the maxillary and premaxillary, in the union of the post-temporal with the skull, in the great reduction of the gill-opening, and in the elongation of the pelvic bones. All these characters in less degree are shown in the Squamipinnes. We have also the reduction and final entire loss of ventral fins, the reduction and loss of the spinous dorsal, the compression and final partial or total fusion of the teeth of the upper jaw, the specialization of the scales, which change from bony scutes into a solid coat of mail on the one hand, and on the other are reduced to thorns or prickles and are finally altogether lost. The number of vertebræ is also progressively reduced until in the extreme forms the caudal fin seems attached to the head, the body being apparently wanting. Throughout the group poisonous alkaloids are developed in the flesh. These may produce the violent disease known as ciguatera, directly attacking the nervous system. See p. 182, vol. I.

The three suborders of plectognathous are easily recognized by external characters. In the Sclerodermi (σκλερός, hard; δέρμα, skin) the spinous dorsal is present and the body is more or less distinctly scaly. The teeth are separate and incisor-like and the form is compressed. In the Ostracodermi (ὀστράκος, a box; δέρμα, skin) there is no spinous dorsal, the teeth are slender, and the body is inclosed in an immovable, bony box. In the Gymnodontes (γυμνός, naked; ὀδούς, tooth) the teeth are fused into a beak like that of a turtle, either continuous or divided by a median suture in each jaw, the spinous dorsal is lost, and the body is covered with thorns or prickles or else is naked.

The Scleroderms.—The Sclerodermi include three recent and one extinct families. Of the recent forms, Triacanthidæ is the most primitive, having the ventral fins each represented by a stout spine and the skin covered with small, rough scales. The dorsal has from four to six stiff spines.

Triacanthodes anomalus is found in Japan, Hollardia hollardi in Cuba. Triacanthus brevirostris, with the first spine very large, is the common hornfish of the East Indies ranging northward to Japan.

The Trigger-fishes: Balistidæ.—The Balistidæ, or trigger-fishes, have the body covered with large rough scales regularly arranged. The first dorsal fin is composed of a short stout rough spine, with a smaller one behind it and usually a third so placed that by touching it the first spine may be set or released. This peculiarity gives the name of trigger-fish as well as the older name of Balistes, or cross-bow shooter. There are no ventral fins, the long pelvis ending in a single blunt spine. The numerous species of trigger-fishes are large coarse fishes of the tropical seas occasionally ranging northward. The center of distribution is in the East Indies, where many of the species are most fantastically marked. Balistes carolinensis, the leather-jacket, or cucuyo, is found in the Mediterranean as also on the American coast. Balistes vetula, the oldwife, oldwench, or cochino, marked with blue, is common in the West Indies, as are several other species, as Canthidermis sufflamen, the sobaco, and the jet-black Melichthys piceus, the black oldwife, or galafata. Several species occur on the Pacific Coast of Mexico, the Pez Puerco, Balistes verres, being commonest. Still others are abundant about the Hawaiian Islands and Japan. The genus Balistapus, having spinous plates on the tail, contains the largest number of species, these being at the same time the smallest in size and the most oddly colored. Balistapus aculeatus and Balistapus undulatus are common through Polynesia to Japan. Most of the tropical species of Balistidæ are more or less poisonous, causing ciguatera, the offensive alkaloids becoming weaker in the northern species. Melichthys radula abounds in Polynesia. In this species great changes take place at death, the colors changing from blue and mottled golden green to jet black. Other abundant Polynesian species are Xanthichthys lineopunctatus, Balistes vidua, Balistes bursa, and Balistes flavomarginatus.

The File-fishes: Monacanthidæ.—Closely related to the Balistidæ are the Monacanthidæ, known as filefishes, or foolfishes. In these the body is very lean and meager, the scales being reduced to shagreen-like prickles. The ventral fins are replaced by a single movable or immovable spine, which is often absent, and the first dorsal fin is reduced to a single spine with sometimes a rudiment behind it. The species are in general smaller than the Balistidæ and usually but not always dull in color. They have no economic value and are rarely used as food, the dry flesh being bitter and offensive. The species are numerous in tropical and temperate seas, although none are found in Europe. On our Atlantic coast, Stephanolepis hispidus and Ceratacanthus schœpfi are common species. In the West Indies are numerous others, Osbeckia lævis and Alutera güntheriana, largest in size, among the commonest. Both of these are large fishes without ventral spine. Monacanthus chinensis, with a great, drooping dewlap of skin behind the ventral spine, is found on the coast of China. Of the numerous Japanese species, the most abundant and largest is Pseudomonacanthus modestus, with deep-blue fins and the ventral spine immovable. Another is Stephanolepis cirrhifer, known as Kawamuki, or skin-peeler. Alutera monoceros, and Osbeckia scripta, the unicorn fish, abound in the East Indies, with numerous others of less size and note. In the male of the Polynesian Amanses scopas (Fig. 347) the tail is armed with a brush of extraordinarily long needle-like spines.

In Stephanolepis spilosomus the caudal fin is of a brilliant scarlet color, contrasting with the usual dull colors of these fishes. In Oxymonacanthus longirostris the body is blue with orange checker-like spots and the snout is produced in a long tube. About the islands of Polynesia, filefishes are relatively few, but some of them are very curious in form or color.

The Spinacanthidæ.—In the extinct family Spinacanthidæ the body is elongate, high in front and tapering behind. The first dorsal has six or seven spines, and there are rough spines in the pectoral. The teeth are bluntly conical. Spinacanthus blennioides and S. imperalis are found in the Eocene of Monte Bolca. These are probably the nearest to the original ancestor among known scleroderms.

The Trunkfishes: Ostraciidæ.—The group Ostracodermi contains the single family of Ostraciidæ, the trunkfishes or cuckolds. In this group, the body is enveloped in a bony box, made of six-sided scutes connected by sutures, leaving only the jaws, fins and tail free. The spinous dorsal fin is wholly wanting. There are no ventral fins, and the outer fins are short and small. The trunkfishes live in shallow water in the tropical seas. They are slow of motion, though often brightly colored.

Against most of their enemies they are protected by the bony case. The species range from four inches to a foot in length, so far as known. They are not poisonous, and are often baked in the shell. Three genera are recognized: Lactophrys with the carapace, three-angled; Ostracion with four angles; and Aracana, resembling Ostracion, but with the carapace not closed behind the anal fin. In each of these genera there is considerable minor variation due to the presence or absence of spines on the bony shell. In some species, called cuckolds, or cowfishes, long horns are developed over the eye. Others have spines on some other part of the shield and some have no spines at all. No species are found in Europe, and none on the Pacific coast of America. The three-angled species, called Lactophrys, are native chiefly to the West Indies, sometimes carried by currents to Guinea, and one is described from Australia. Lactophrys tricornis of the West Indies has long horns over the eye; Lactophrys trigonus has spines on the lower parts only. Lactophrys triqueter is without spines, and the fourth American species, Lactophrys bicaudalis, is marked by large black spots. The species of Ostracion radiate from the East Indies. One of them, Ostracion gibbosum, has a turret-like spine on the middle of the back, causing the carapace to appear five-angled; Ostracion diaphanum has short horns over the eye, and Ostracion cornutum very long ones; Ostracion immaculatus, the common species of Japan, is without spines; Ostracion sebæ of Hawaii and Samoa is deep, rich blue with spots of golden. Aracana is also of East Indian origin; Aracana aculeata, with numerous species, is common in Japan. A fossil species of Ostracion (O. micrurum) is known from the Eocene of Monte Bolca.

The Gymnodontes.—The group of Gymnodontes, having the teeth united in a turtle-like beak, carry still further the degeneration of scales and fins. There is no trace of spinous dorsal, or ventral. The scales are reduced to thorns or prickles, or are lost altogether. All the species have the habit of inflating themselves with air when disturbed, thus floating, belly upward, on the surface of the water. Very few, and these only northern species, are used as food, the flesh of the tropical forms being generally poisonous, and that often in a higher degree than any other fishes whatever.

The Triodontidæ.—The most generalized family is that of the Triodontidæ. These fishes approach the Balistidæ in several regards, having the body compressed and covered with rough scales. The teeth form a single plate in the lower jaw, but are divided on the median line above. The compressed, fan-like, ventral flap is greatly distensible. Triodon bursarius, of the East Indies and northward to Japan, is the sole species of the family.

The Globefishes: Tetraodontidæ.—In the Tetraodontidæ (globefishes, or puffers), each jaw is divided by a median suture. The dorsal and anal are short, and the ventrals are reduced in number, usually fifteen to twenty (7 + 13 to 7 + 9). The walls of the belly are capable of extraordinary distension, so that when inflated, the fish appears like a globe with a beak and a short tail attached. The principal genus Spheroides contains a great variety of forms, forming a closely intergrading series. In some of these the body is smooth, in others more or less covered with prickles, usually three-rooted. In some the form is elongate, the color silvery, and the side of the belly with a conspicuous fold of skin. In these species, the caudal is lunate and the other fins falcate, and with numerous rays. But these forms (called Lagocephalus) pass by degrees into the short-bodied forms with small rounded fins, and no clear line has yet been drawn separating the group into genera. In these species each nostril has a double opening. Lagocephalus lagocephalus, large and silvery, is found in Europe. Lagocephalus lævigatus replaces it on the Atlantic Coast of North America. In Japan are numerous forms of this type, the venomous Lagocephalus sceleratus being one of the best known. Numerous other Japanese species, Spheroides xanthopterus, rubripes, pardalis, ocellatus, vermiculatus, chrysops, etc., mark the transition to typical Spheroides. Spheroides maculatus is common on our Atlantic coast, the puffer, or swell-toad of the coastwise boys who tease it to cause it to swell. Spheroides spengleri and S. testudineus abound in the West Indies. Spheroides politus on the west coast of Mexico.

In Tetraodon the nasal tentacle is without distinct opening, its tip being merely spongy. The species of this genus are even more inflatable and are often strikingly colored, the young sometimes having the belly marked by concentric stripes of black which disappear with age. Tetraodon hispidus abounds in estuaries and shallow bays from Hawaii to India. In Hawaii, it is regarded as the most poisonous of all fishes (muki-muki) and it is said that its gall was once used to poison arrows. Tetraodon fahaka is a related species, the first known of the family. It is found in the Nile. Tetraodon lacrymatus, black with white spots, is common in Polynesia. Tetraodon aërostaticus, with black spots, is frequently taken in Japan, and Tetraodon setosus is frequent on the west coast of Mexico. This species is subject to peculiar changes of color. Normally dark brown, with paler spots, it is sometimes deep blue, sometimes lemon-yellow and sometimes of mixed shades. Specimens showing these traits were obtained about Clarion Island of the Revillagigedos. No Tetraodon occurs in the West Indies. Colomesus psittacus, a river fish of the northern part of South America, resembles Spheroides, but shows considerable difference in the skull.

But few fossil Tetraodontidæ have been recognized. These are referred to Tetraodon. The earliest is Tetraodon pygmæus from Monte Bolca.

The Chonerhinidæ of the East Indies are globefishes having the dorsal and anal fins very long, the vertebræ more numerous (12 + 17), twenty-nine in number. Chonerhinus naritus inhabits the rivers of Sumatra and Java.

The little family of Tropidichthyidæ is composed of small globefishes, with a sharply-keeled back, and the nostrils almost, or quite, wanting. The teeth are as in the Tetraodontidæ. The skeleton differs considerably from that of Spheroides, apparently justifying their separation as a family. The species are all very small, three to six inches in length, and prettily colored. In the West Indies Tropidichthys rostratus is found. Tropidichthys solandri abounds in the South Seas, dull orange with blue spots. Tropidichthys rivulatus is common in Japan and several ether species are found in Hawaii.

Other species occur on the west coast of Mexico, in Polynesia, and in the East Indies.

The Porcupine-fishes: Diodontidæ.—In the remaining families of Gymnodontes, there is no suture in either jaw, the teeth forming an undivided beak. The Diodontidæ, or porcupine-fishes, have the body spherical or squarish, and armed with sharp thorns, the bases of which are so broad as to form a continuous coat of mail. In some of them, part of the spines are movable, these being usually two-rooted; in others, all are immovable and three-rooted. All are reputed poisonous, especially in the equatorial seas.

In Diodon the spines are very long, the anterior ones, at least, movable. The common porcupine-fish, Diodon hystrix, is found in all seas, and often in abundance. It is a sluggish fish, olive and spotted with black. It reaches a length of two feet or more, and by its long spines it is thoroughly protected from all enemies. A second species, equally common, is the lesser porcupine-fish, Diodon holacanthus. In this species, the frontal spines are longer than those behind the pectoral, instead of the reverse, as in Diodon hystrix. Many species of Diodon are recorded from the Eocene, besides numerous species from later deposits. One of these, as Heptadiodon heptadiodon from the Eocene of Italy, with the teeth subdivided, possibly represents a distinct family. Diodon erinaceus is found in the Eocene of Monte Bolca and Progymnodon hilgendorfi in the Eocene of Egypt.

In the rabbit-fishes (Chilomycterus) the body is box-shaped, covered with triangular spines, much shorter and broader at base than those of Diodon. Numerous species are known.

Chilomycterus schœpfi is the common rabbit-fish, or swell-toad of our Atlantic coast, light green, prettily varied with black lines. The larger, Chilomycterus affinis, with the pectoral fin spotted with black, is widely diffused through the Pacific. It is rather common in Japan, where it is the torabuku, or tiger puffer. It is found also in Hawaii, and it is once recorded by Dr. Eigenmann from San Pedro, California, and once by Snodgrass and Heller, from the Galapagos.

The Head-fishes: Molidæ.—The headfishes, or Molidæ, also called sunfishes, have the body abbreviated behind so that the dorsal, anal, and caudal fins seem to be attached to the posterior outline of the head. This feature, constituting the so-called gephyrocercal tail is a trait of specialized degradation.

Mola mola, the common head-fish or sunfish, is found occasionally in all tropical and temperate seas. Its form is almost circular, having been compared by Linnæus to a mill-wheel (mola), and its surface is covered with a rough, leathery skin. It swims very lazily at the surface of the water, its high dorsal often rising above the surface. It is rarely used as food, though not known to be poisonous. The largest example known to the writer was taken at Redondo Beach, California, by Mr. Thomas Shooter, of Los Angeles. This specimen was 8 feet 2 inches in length, and weighed 1200 pounds. Another, almost as large, was taken at San Diego, in April, 1904. No difference has been noticed among specimens from California, Cape Cod, Japan, and the Mediterranean. The young, however, differ considerably from the adult, as might be expected in a fish of such great size and extraordinary form. (See Figs. 109 and 110, Vol. I.)

Fragments named Chelonopsis, and doubtfully referred to Mola, are found in the Pliocene of Belgium. Certain jaws of cretaceous age, attributed to Mola, probably belong, according to Woodward, to a turtle.

In the genus Ranzania, the body is more elongate, twice as long as deep, but as in Mola, the body appears as if bitten off and then provided with a fringe of tail. The species are rarely taken. Ranzania truncata is found in the Mediterranean and once at Madeira. Ranzania makua, known as the king of the mackerels about Hawaii, is beautifully colored brown and silvery. This species has been taken once in Japan.

In Hawaii it is believed that all the Scombroid fishes are subject to the rule of the makua and that they will disappear if this fish be killed. By a similar superstition, Regalecus glesne is "king of the herrings" in Norway and about Cape Flattery, Trachypterus rex salmonorum is "king of the salmon."