Page:An Introduction to the Study of Fishes.djvu/99

Rh the dorsal fin is armed articulates with a neural apophysis, and is not immovably attached to it, as in the Sharks. The pubic consists of two lateral halves, with a short, rounded, tarsal cartilage.

The skeleton of the Ganoid Fishes offers extreme variations with regard to the degree in which ossifications replace the primordial cartilage. Whilst some exhibit scarcely any advance beyond the Plagiostomes with persistent cartilage, others approach, as regards the development and specialisation of the several parts of their osseous framework, the Teleosteans so closely that their Ganoid nature can be demonstrated by, or inferred from, other considerations only. All Ganoids possess a separate gill-cover.

The diversity in the development of the Ganoid skeleton is well exemplified by the few representatives of the order in the existing Fish-fauna. Lowest in the scale (in this respect) are those with a persistent notochord, and an autostylic skull, that is, a skull without separate suspensorium—the fishes constituting the sub-order Dipnoi, of which the existing representatives are Lepidosiren, Protopterus, and Ceratodus, and the extinct (as far as demonstrated at present) Dipterus, Chirodus (and Phaneropleuron ?). In these fishes the notochord is persistent, passing uninterruptedly into the cartilaginous base of the skull. Only now and then a distinct vertical segmentation occurs in the caudal portion of the column, but it does not extend to the notochord itself, but indicates only the limits between the superadded apophyseal elements, each neural being confluent with the opposite hæmal. Some Dipnoi are diphy-, others hetero-cercal.