Page:An Introduction to the Study of Fishes.djvu/100

Neural and hæmal elements and ribs are well developed. In Ceratodus each neurapophysis consists of a basal cartilaginous portion, forming an arch over the myelon, and of a superadded second portion. The latter is separated from the former by a distinct line of demarcation, and its two branches are more styliform, cartilaginous at the ends and in the centre, but with an osseous sheath, and coalesced at the top, forming a gable over an elastic fibrous band which runs along and parallel to the longitudinal axis of the column (Ligamentum longitudinale superius). To the top of this gable is joined a single long cylindrical neural spine. From the eleventh apophyseal segment a distinct interneural spine, of the same structure as the neural, begins to be developed, and farther on a second interneural is superadded. Towards the extremity of the column these various pieces are gradually reduced in size and number, finally only a low cartilaginous band (the rudiments of the neurapophysis) remaining. The hæmapophyses are in form, size, and structure, very similar to the neurapophyses; and all these long bones, including the ribs, have that in common, that they consist of a solid rod of cartilage enclosed in a bony sheath, which, after the disappearance or decomposition of the cartilage, appears as a hollow tube. Such bones are extremely common throughout the order of Ganoids, and their remains have led to the designation of a family as Cœlacanthi (κοιλος, hollow; and ἀκανθος, spine).

The primordial cranium of the Dipnoi is cartilaginous, but with more or less extensive ossifications in its occipital, basal, or lateral portions, and with large tegumentary bones, the arrangement of which varies in the different genera. There is no separate suspensorium for the lower jaw. A strong process descends from the cranial cartilage, and offers by means of a double condyle (Fig. 35 s) attachment to corresponding articulary surfaces of the lower jaw.