DIPNOI, one of the primary subdivisions of fishes, characterized by the presence of mem brane or covering bones on the skull, the ab sence of secondary bony jaws (maxillary, pre maxillary and dentary bones), and the direct articulation of the lower jaw with the skull. The Dipnoi were early predominant in geologi cal times, many and varied forms occurring as fossils in the Middle and Upper Palaeozoic strata. Some of these, forming the order Ar throdira, are of doubtful relationship, and are classed with the Dipnoi only provisionally. The best-known representatives of this group have the anterior region of the body enclosed in continuous bony plates, with a complex articu lation between the head and trunk. In the De vonian of Ohio are found giant forms, like Dinichthys and Macropetalichthys, from 10 to 25 feet long, and with sharp-beaked jaws like a turtle's. Numerous other Dipnoi, more closely resembling the still living ones, have been found as fossils in various parts of the world. At the present time the group is decadent, and but three genera and as many species, each found in a restricted area in widely separated regions of the earth, remain. They are, Protopterus
from Africa, Lepidosiren from South America, and Ceratodus from Australia. On account of the lung-like structure of the swim-bladder and its function as an organ of respiration, they are known as lung-fishes. Being the earliest verte brates which are believed to have breathed air, the Dipnoi have an exceptional interest, and many zoologists consider them to been the original stock from which the Amphsbta and all other terrestrial vertebrates arose. See MUD FISH; STEGOCEPHALIA. Consult Cope, of the Fittest' (New York 1886) ; id., (Batra chia of North America' (Washington 1899) ; Dean, (Fishes, Living and Fossil' (New York 1895) ; Gadow, (Amphibia) (London 1901) ; Lydekker, Cunningham, Boulenger and Thom son, (Reptiles, Amphibia, and Fishes' (ib. 1912).