The developed lung, fitted for breathing air, which seems the most important of all these characters, can, of course, be traced only in the recent forms, although its existence in all others can be safely predicated. Besides the de velopment of the lung we may notice the gradual forward movement of the shoulder girdle, which, in the dipneustans, as in the crossopterygians, is attached to the head. In the fishes generally there is no distinct neck, as the post-temporal, the highest bone of the shoulder-girdle, is articulated directly with the skull.
We may divide the dipnoans into two orders, Ctenodipterini, with the families Uronenside (extinct), Dipterider ( extinct ), and Ctenodon tide (extinct). These families occur from the Devonian to the Mesozoic. The more specialized order of Sirenoidei includes the families of Ceratodontide and Lepidosirenide, each of these represented.by living forms. Most of the Ceratodontide are extinct, occurring in the Mesozoic; but two species, Neoceratodus forsteri and N. miolepis, live in rivers of Aus tralia. No fossil Lepidosirenida are known. Lepidosiren paradosus lives in the swamps of southern Brazil and Protopterus onnectens, P. clonal, and P. tethopicus, in those of the Nile region.
Sub-class Actinopteri—After setting off from the great group of fishes primitive or archaic types, one after another, we are left at last with only those having fish-jaws, fish-fins, and in general the structure of the typical fish. For all these in all their variety, as a class or sub-class, we adopt the name of Actinopteri, suggested by Prof. Cope. The name (iurris, ray; imply, fin) refers to the structure of the paired fins. In all these, the bones supporting the fin rays are highly specialized, and at the same time concealed by the general integument of the body.
In general, two bones connect the pectoral fin with the shoulder-girdle. The hyperacora coid is a flat square bone, usually perforated by a foramen lying above, and parallel with it the irregularly formed hypocoracoid. Attached to these is a row of bones, the actinosts or ptery gials, short, often hourglass-shaped, which actu ally support the fin-rays. In the higher forms the actinosts are few (four to six) in number, but in the lower types they may remain numer ous, a reminiscence of the condition seen in the crossopterygians and especially in Polyp terus. Other variations may occur; the two
coracoids are sometimes imperfect or specially modified, and the actinosts may be distorted in form or position. Among the lower Actinopteri many archaic traits still persist, and by its earlier representatives the group is joined very closely to the Crossopterygii. The great class may be divided into two series or sub-classes, the Ganoidei, which retain ancient traits, and the Teleostei or bony fishes, in which most of these have disappeared.
Even among the Ganoidei, as the term is here restricted, there remains a great variety of form and structure. The group constitutes sev eral distinct orders, and as a whole does not admit of perfect definition. All of the species known have the tail strongly heterocercal. Most of them have the skeleton still cartilaginous, and in some it remains in a very primitive con dition. Most of them have an armature of bony plates, diamond-shaped with an enamel like the surface of teeth. All of them have the air bladder highly developed, usually cellular and functional as a lung, hut connecting with the dorsa side of the gullet, not with the ven tral side, as in the dipnoans. In all these re main more or less perfectly developed the optic chiasma, the many valves of the arterial bulb. and the spiral valve of the intestines found in the more archaic types. But traces of some or all of these structures are found in some bony fishes, and their presence in the ganoids by no means justifies their separation with sharks, dip noans, and crossopterygians as a great primary class, Pakeichthyes as proposed by Dr. Gunther. All forms of body may be found among the ganoids. In the earlier seas they were scarcely less varied and perhaps scarcely less abundant that the teleosts in the seas of to-day. So far as fossils show, the characteristic actinop terous fin, with its reduced and altered basal bones, appeared• at once without intervening gradations.
The name Ganoidei (yevoc brightness; eicki, resemblance), alluding to the enameled plates, was first given by Agassiz to those forms, mostly extinct, allied to the garpike, and cov ered with bony scales or hard plates. As originally defined cat-fishes, sea-horses, Agoni da., and other wholly unrelated types were in cluded with the garpikes and sturgeons as ganoids.