Page:An Introduction to the Study of Fishes.djvu/105

Rh corner of a basal cartilage (a) a short axis (b) is joined, which on its outer side bears a few branches (d) only, the remaining branches (c) being fixed to the basal cartilage. The dermal fin-rays are opposed to the extremities of the branches, as in the Dipnoi.

The pubic consists of a paired cartilage, to which tarsal pieces supporting the fin-rays are attached.

The other living Ganoid fishes have the spinous column entirely or nearly entirely ossified, and have been comprised under the common name Holostei. However, they form three very distinct types; several attempts have been made to coordinate with them the fossil forms, but this task is beset with extreme difficulties, and its solution hitherto has not proved to be satisfactory.

The Polypteroidei have their spinous column formed by distinct osseous amphicœlous vertebræ, that is, vertebræ with concave anterior and posterior surfaces. It is nearly diphycercal; a slight degree of heterocercy obtains, inasmuch as the last vertebra is succeeded by a very thin cartilaginous filament which penetrates between the halves of one of the middle rays of the terminal fin. The rays above this cartilaginous filament are articulated to interneurals, those below lack interhæmals, and are attached either to the hæmals or vertebral centres. The neural arches, though ossified, do not coalesce with the centrum, and form one canal only, for the myelon. There are no intermediate elements between the neural spines. Interneurals developed, but simple, articulating with the dermoneurals. The abdominal vertebræ have parapophyses developed with epipleural spines. Only the caudal vertebræ have hæmal spines, which, like the interhæmals, agree in every essential respect with the opposite neurals. Ribs are inserted, not on the parapophyses, but on the centre, immediately below the parapophyses.

The skull of Polypterus (Fig. 39) shows a great advance