The single family, _Siganidæ_, contains fishes of moderate size, valued as food, and abounding about rocks in shallow water from the Red Sea to Tahiti. The coloration is rather plain olive or brown, sometimes with white spots, sometimes with bluish lines. The species are very much alike and all belong to the single genus _Siganus_. One species, _Siganus fuscescens_, dusky with small, pale dots, is a common food-fish of Japan. Others, as _Siganus oramin_ and _Siganus vermiculatus_, occur in India, and _Siganus punctatus_, known as lo, abounds about the coral reefs of Samoa. _Siganus vulpinus_ differs from the others in the elongate snout. A fossil genus, _Archoteuthis_ (_glaronensis_), is found in the Tertiary of Glarus. It differs from _Siganus_ in the deeper body and in the presence of six instead of seven spines in the anal fin. The real relationship of the _Siganidæ_ is still uncertain, but the family is probably most nearly allied to the _Acanthuridæ_, with which the species were first combined by Linnæus, who included both in his genus _Teuthis_. In the structure of the vertical fins the _Siganidæ_ resemble the extinct genus _Pygæus_. CHAPTER XXIV SERIES PLECTOGNATHI =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.