This order is a small one, and includes no other living forms than Mud-fishes (Lepidasiren and Ceratodus); but it is nevertheless of great importance as exhibiting a distinct transition between the fishes and the Amphibia. So many, in fact, and so striking, are the points of resemblance between the two, that until recently the Lepidosiren (fig. 272) was always made to constitute the lowest class of the Amphibia. The highest authorities, however, now concur in placing it amongst the fishes, of which it constitutes, with Ceratodus, the highest order. The order Dipnoi is defined by the following characters: The body is fish - like in shape. There is a skull with distinct cranial bones and a lower jaw, but the notochord is persistent, and there are no vertebral centra, nor an occipital condyle. The exoskeleton consists, in the living types, of horny overlapping scales, having the "cycloid" character; but various extinct forms have "ganoid" scales. The pectoral and ventral limbs are both present, but have (in Lepidosiren) the form of awl-shaped, filiform, many-jointed organs, of which the former only have a membranous fringe inferiorly. The ventral limbs are attached close to the anus, and the pectoral arch has a clavicle ; but the scapular arch is attached to the occiput. In Ceratodus (fig. 273) the pectoral and ventral limbs have the same form as in the Crossopterygious Ganoids. The caudal fin is symmetrical, or, in some extinct forms, may be heterocercal. The heart has two auricles and one ventricle. The respiratory organs are twofold, consisting, on the one hand, of free filamentous gills contained in a branchial chamber, which opens externally by a single vertical gill-slit; and, on the other hand, of true lungs in the form of a double cellular air-bladder, communicating with the oesophagus by means of an air-duct or trachea. The branchiae are supported upon branchial arches, but these are not connected with the hyoid bone ; and in some cases, at any rate, rudimentary external branchial exist as well. The nasal sacs open posteriorly into the throat.
Fig. 272. - Dipnoi. Lepidosiren annectens.
If these characters are examined a little more minutely, it is easy to point to those in which the Dipnoi approach the Fishes, and to those in which they resemble the Amphibians. They resemble the fishes in the shape of the body, and in the possession of a covering of horny overlapping scales of the true cycloid character; whilst the limbs are more like those of fishes than of reptiles. The fin, also, which clothes the posterior extremity of the body, is of a decided fish-like character. The most marked piscine feature, however, is the presence of free branchiae, attached to branchial arches, and placed in a branchial cavity, which opens internally into the pharynx by a number of fissures, and communicates externally with the outer world by means of a single vertical gill-slit.
On the other hand, the Dipnoi approximate to the Amphibians in the following important points: The heart consists of three cavities, two auricles, and a single ventricle. True lungs are present, with a trachea and glottis, returning their blood to the heart by a distinct pulmonary vein, and in every respect discharging the functions of the lungs of the higher Vertebrates. It is true that the lungs of the Dipnoi are merely a modification of the swim-bladder of the other fishes, but the significance of the change of function is not affected by this. Lastly, sometimes, at any rate, there are rudimentary external branchiae placed on the side of the neck. This feature, as will be seen shortly, is characteristic of all the Amphibians, either permanently or in their immature state.
Upon the whole, then, whilst for the purposes of systematic classification the Dipnoi must be placed amongst the Fishes, it is not to be forgotten that many of their characters are those of a higher class, and that they may justly be looked upon as a connecting link, or transitional group, between the two great divisions of the Fishes and the Amphibians.
As regards their distribution and mode of life, two species at least of Lepidosiren are known - the L. paradoxa from the Amazon, and the L. (Protopterus) annectens from the Gambia. They both inhabit the waters of marshy tracts, and are able in the dry season to bury themselves in the mud, forming a kind of chamber, in which they remain dormant till the return of the rains. Recently there has been discovered in the rivers of Queensland (Australia) a fish which has been described under the name of Ceratodus Forsteri, and which shows itself to be very closely related to the Lepidosiren. This singular fish (fig. 273) - the "Jeevine" * of the natives - is from three to six feet long, and has the body covered with large cycloid scales, a species with smaller scales having been described as C. mio-lepis. The skeleton is notochordal, all the bones remaining permanently cartilaginous. There is a well-developed operculum, but - as in Lepidosiren - no branchiostegal rays. The tail is homocercal, and the pectoral and ventral fins are supported by a median, many-jointed, cartilaginous rod, with numerous lateral branches on each side. The heart consists of a single auricle and ventricle, with a "Ganoid" bulbus arteriosus. There are five branchial arches, of the Teleostean type, but cartilaginous. The swim-bladder is single, composed of two symmetrical halves, cellular in structure, with a pneumatic duct and glottis, as in Lepidosiren. The intestine has a spiral valve, and there are no pyloric caeca. There are two molar teeth in each jaw, having the form of flattened undulated plates of bone, singularly like the teeth of Ceratodus from the Trias (fig. 274). The Ceratodi employ these crush ing teeth in the mastication of vegetable matter, upon which they feed; and they are stated to leave the streams which they inhabit, at night time, in order to betake themselves to the marshy flats in the vicinity, where they obtain an abundant supply of food.
Fig. 273. - Ceratodus Forsteri, the Australian Mud-fish, reduced in size.
* The name of "Barramunda " seems to have been given to Ceratodus by error, the native Australians apparently calling it the "Jeevine" or "Teebine."
Fig. 274. - A, Dental plate of Ceratodus serratus, Keuper. B, Dental plate of Ceratodus alius, Keuper. (After Agassiz.)
The genera Lepidosiren (with Protopterus) and Ceratodus comprise the few living species of Dipnoi, and constitute a special division (Sirmoidei) characterised by the possession of horny cycloidal scales, a symmetrically divided caudal fin, and an undivided dorsal fin. There are, however, a few Palaeozoic genera of Dipnoi - of which the most important are Dipterus and Ctenodus - which constitute a distinct sub-order (Ctenodip-terini), characterised by their enamelled scales, their hetero-cercal tails, and their possession of two dorsal fins.
Upon the whole, Dr Gunther concludes that the Dipnoi are to be regarded as a simple sub-order of Ganoids, and that the entire order Ganoidei may be united with Elasmobranchii into a single order, called Palaeichthyes, characterised by having a "heart, with a contractile bulbus arteriosus, intestine, with a spiral valve, and optic nerves non-decussating."