[Illustration: Fig. 137.--Coccosteus, after Pander. _A_, Anal fin; _D_, Dorsal fin; _C_, Heterocercal tail; _c_, notochord; _n_, neurals; _h_, hæmals; 6–24, bucklers.] SECOND SUB-ORDER--ACANTHODINI. Extinct. _Body oblong, compressed, covered with shagreen; skull not ossified; caudal heterocercal. Large spines, similar to those of Chondropterygians, in front of some of the median and paired fins. The spines are imbedded between the muscles, and not provided with a proximal joint._ _Acanthodes_, _Chiracanthus_, from Devonian and Carboniferous formations. THIRD SUB-ORDER--DIPNOI. _Nostrils two pairs, more or less within the mouth; limbs with an axial skeleton. Lungs and gills. Skeleton notochordal. No branchiostegals._[38] FIRST FAMILY--SIRENIDÆ. _Caudal fin diphycercal; no gular plates; scales cycloid. A pair of molars, above and below, and one pair of vomerine teeth._ LEPIDOSIREN.--Body eel-shaped, with one continuous vertical fin. Limbs reduced to cylindrical filaments, without fringe. Vomerine teeth conical, pointed. Each dental lamina or molar with strong cusps, supported by vertical ridges. No external branchial appendages; five branchial arches, with four intervening clefts. Conus arteriosus with two longitudinal valves. Ovaries closed sacs. One species only is known from the system of the River Amazons (_L. paradoxa_). It must be very locally distributed, as but a few specimens have been brought to Europe, and all recent endeavours to obtain others have been unsuccessful. Natterer, by whom this most interesting fish was discovered, states that he obtained two specimens, one on the Madeira River, near Borba; the other in a backwater of the Amazons, above Villa Nova. The natives of the former place called it Carámurú, and considered it very scarce. The larger individual was nearly four feet long. It is said to produce a sound not unlike that of a cat, and to feed on the roots of mandioca and other vegetables. But, to judge from the dentition, this fish is much more likely to be carnivorous, like the following. It is one of the greatest desiderata of Natural History Collections. [Natterer, “Annalen des Wiener Museum’s,” 1839, ii.; Bischoff. “Annales des Sciences Naturelles,” 1840. xiv.] PROTOPTERUS.--Very similar in the general form of the body and dentition to _Lepidosiren_. Pectoral and ventral filaments with a fringe containing rays. Three small branchial appendages above the small gill-opening; six branchial arches, with five intervening clefts. Conus arteriosus with two longitudinal valves. Ovaries closed sacs.