Page:An Introduction to the Study of Fishes.djvu/101

Rh Maxillary and intermaxillary elements are not developed, but, perhaps, represented in Ceratodus by some inconstant mdimentary labial cartilages situated behind the posterior nasal opening. Facial cartilages and an infraorbital ring are developed at least in Ceratodus. The presence of a pair of small teeth in front indicates the vomerine portion (v) which remains cartilage, whilst the posterior pair of teeth are implanted in a pterygo-palatine ossification (l), which sometimes is paired, sometimes continuous. The base of the skull is constantly covered by a large basal ossification (o).

The hyoid is well developed, sometimes reduced to a pair of ceratohyals, sometimes with a basihyal and glossyhyal. The skeleton of the branchial apparatus approaches the Teleosteous type, less so in Lepidosiren than in Ceratodus, in which five branchial arches are developed, but with the lateral and mesial pieces reduced in number.

A large operculum, and a smaller sub- or inter-operculum are present.

The scapular arch consists of a single median transverse cartilage, and a pair of lateral cartilages which bear the articular condyle for the pectoral limb. The latter cartilages form the base of a large membrane-bone, and the whole arch is suspended from the skull by means of an osseous supraclavicle.

The fore-limb of the Dipnoi (Fig. 36) differs externally greatly from the pectoral fin of other Ganoid fishes. It is covered with small scales along the middle, from the root