Fig. 371.—Arm of a frog.
The reptiles differ from the Amphibians through acceleration of development, passing through the gill-bearing stages within the egg. The birds bear feathers instead of scales, and the mammals nourish their young by means of glandular secretions. Through a reptile-amphibian ancestry the birds and mammals may trace back their descent from palæozoic Crossopterygians. In the very young embryo of all higher vertebrates traces of double-breathing persist in all species, in the form of rudimentary gill-slits.
The Fins of Crossopterygians.—Dollo and Boulenger regard the heterocercal tail as a primitive form, the diphycercal form being a result of degradation, connected with its less extensive use as an organ of propulsion. Most writers who adopt the theory of Gegenbaur that the archipterygium is the primitive form of the pectoral fin are likely, however, to consider the diphycercal tail found associated with it in the Ichthyotomi, Dipneusti, Crossopterygii as the more primitive form of the tail. From this form the heterocercal tail of the higher sharks and Ganoids may be derived, this giving way in the process of development to the imperfectly homocercal tail of the salmon, the homocercal tail of the perch, and the isocercal tail of the codfish and its allies, the gephyrocercal and the leptocercal tail, tapering or whip-like, representing various stages of degeneration. Boulenger draws a distinction between the protocercal tail, the one primitively straight, and the diphycercal tail modified, like the homocercal tail, from an heterocercal ancestry.
Fig. 372.—Polypterus congicus, a Crossopterygian fish from the Congo River. Young, with external gills. (After Boulenger.)
Orders of Crossopterygians.—Cope and Woodward divide the Crossopterygia into four orders or suborders, Haplistia, Rhipidistia, Actinistia, and Cladistia. To the latter belong the existing species, or the family of Polypteridæ, alone. Boulenger unites the three extinct orders into one, which he calls Osteolepida. In all three of these the pectorals are narrow with a single basal bone, and the nostrils, as in the Dipneustans, are below the snout. The differences are apparently such as to justify Cope's division into three orders.
Haplistia.—In the Haplistia the notochord is persistent, and the basal bones of dorsal and anal fins are in regular series, much fewer in number than the fin-rays. The single family Tarrassiidæ is represented by Tarrasius problematicus, found by Traquair in Scotland. This is regarded as the lowest of the Crossopterygians, a small fish of the Lower Carboniferous, the head mailed, the body with small bony scales.
Rhipidistia.—In the Rhipidistia the basal bones of the median fins ("axonosts and baseosts") are found in a single piece, not separate as in the Haplistia. Four families are recognized, Holoptychiidæ, Megalichthyidæ, Osteolepidæ, and Onychodontidæ, the first of these being considered as the nearest approach of the Crossopterygians to the Dipnoans.
The Holoptychiidæ have the pectoral fins acute, the scales cycloid, enameled, and the teeth very complex. Holoptychius nobilissimus is a very large fish from the Devonian. Glyptolepis leptopterus from the Lower Devonian is also a notable species. Dendrodus from the Devonian is known from detached teeth.