=Order Lysopteri.=—In the most primitive order, called _Lysopteri_ (λυσός, loose; πτερόν, fin) by Cope, _Heterocerci_ by Zittel and Eastman, and the "ascending series of Chondrostei" by Woodward, we find the nearest approach to the Chondropterygians. In this order the arches of the vertebræ are more or less ossified, the body is more or less short and deep, covered with bony dermal plates. The opercular apparatus is well developed, with numerous branchiostegals. Infraclavicles are present, and the fins provided with fulcra. Dorsal and anal fins are present, with rays more numerous than their supports; ventral fin with basal supports which are imperfectly ossified; caudal fin mostly heterocercal, the scales mostly rhombic in form. All the members of this group are now extinct. =The Palæoniscidæ.=—The numerous genera of this order are referred to three families, the _Palæoniscidæ_, _Platysomidæ_, and _Dictyopygidæ_; a fourth family, _Dorypteridæ_, of uncertain relations, being also tentatively recognized. The family of _Palæoniscidæ_ is the most primitive, ranging from the Devonian to the Lias, and some of them seem to have entered fresh waters in the time of the coal-measures. These fishes have the body elongate and provided with one short dorsal fin. The tail is heterocercal and the body covered with rhombic plates. Fulcra or rudimentary spine-like scales are developed on the upper edge of the caudal fin in most recent Ganoids, and often the back has a median row of undeveloped scales. A multitude of species and genera are recorded. A typical form is the genus _Palæoniscum_,[5] with many species represented in the rocks of various parts of the world. The longest known species is _Palæoniscum frieslebenense_ from the Permian of Germany and England. _Palæoniscum magnum_, sixteen inches long, occurs in the Permian of Germany. From _Canobius_, the most primitive genus, to _Coccolepis_, the most modern, is a continuous series, the suspensorium of the lower jaw becoming more oblique, the basal bones of the dorsal fewer, the dorsal extending farther forward, and the scales more completely imbricate. Other prominent genera are _Amblypterus_, _Eurylepis_, _Cheirolepis_, _Rhadinichthys_, _Pygopterus_, _Elonichthys_, _Ærolepis_, _Gyrolepis_, _Myriolepis_, _Oxygnathus_, _Centrolepis_, and _Holurus_. Footnote 5: This word is usually written _Palæoniscus_, but Blainville, its author (1818), chose the neuter form. [Illustration: FIG. 2.—_Palæoniscum frieslebenense_ Blainville. Family _Palæoniscidæ_. (After Zittel.) ] =The Platysomidæ.=—The _Platysomidæ_ are different in form, the body being deep and compressed, often diamond-shaped, with very long dorsal and anal fins. In other respects they are very similar to the _Palæoniscidæ_, the osteology being the same. The _Palæoniscidæ_ were rapacious fishes with sharp teeth, the _Platysomidæ_ less active, and, from the blunter teeth, probably feeding on small animals, as crabs and snails.