Fig. 108.—Modern Dipnoi.
a, Ceratodus Fosteri. Australia. b, Lepidosiren annectus. Africa.
A new and interesting light has recently been cast upon some of the most anomalous of the ancient fishes by the study of the now rare and peculiar species of the group of Dipnoi. Two of these, belonging to the genus Lepidosiren, are the “Mud-fishes” of the rivers of tropical Africa and America ([Fig. 108], b.) These creatures have an elongated and elegant form, and the body is covered with overlapping horny scales like those of ordinary fishes; but the pectoral and ventral fins are rod-like, and are supported by simple cartilaginous rays, while the tailfin forms a fringe around the posterior part of the body. Unlike ordinary fishes, they have lungs as well as gills, and their mouths are armed with sharp, bony, beak-like teeth ([Fig. 115]), with which they can inflict terrible bites on the small fishes and frogs which furnish them with food. Their most remarkable habit is that of burying themselves in the mud of dried-up ponds, thus forming a sort of water-chamber or “cocoon,” in which they remain in a torpid state until the return of the rainy season sets them free.
Another example of these Dipnoi is the Barramunda, or Ceratodus of the Australian rivers ([Fig. 108]a). This fish resembles the Lepidosiren in many essential points of structure; but its fins have lateral rays, and are consequently of some breadth, though of peculiar form, and its mouth is armed with flat, pavement-like teeth, wherewith it browses on aquatic grasses.
Fig. 109.—Anterior part of the palate of Dipterus. Showing the dental plates at a, Devonian.—After Traquair.
These modern fishes have enabled us to understand several mysterious forms met with in the older rocks. In the first place, they show the meaning of certain flat-toothed fishes, like Dipterus of the Devonian ([Fig. 109]), Conchodus of the Carboniferous ([Fig. 110]), and Ceratodus of the Carboniferous and Trias ([Figs. 111, 112]), previously of very doubtful character. These must all have been of similar structure and habits with the Barramunda, which is thus the sole survivor, perhaps itself verging on extinction, of a group of herbivorous fishes introduced, it may be, contemporaneously with the first stream affording the requisite vegetable food, and which have continued almost without improvement or deterioration to the present time. These fishes are, however, very closely connected with the Ganoids, and there are some of these, with fringed fins and overlapping scales, which, while regarded as true Ganoids, resemble the Dipnoi very closely.
