Albula is represented in the Eocene (London Clay and Bruxellian); and the Cretaceous Istieus and Anogmius are believed to be possibly related to Pterothrissus. Four Cretaceous types (Plethodus, Thryptodus, Syntegmodus, and Ancylostylus) are referred with doubt to the Albulidae.
Fam. 7. Mormyridae.—Margin of the upper jaw formed by the single praemaxillary and the maxillaries, the latter articulated above the former to the ethmoid. Parietal bones separating the supraoccipital from the frontals; a large hole on each side of the skull, between the squamosal, the epiotic, and the opisthotic, covered by a large, thin, bony plate (the supratemporal), which may extend over a part of the parietal; symplectic absent; suboperculum small and hidden under the operculum, or absent; interoperculum well developed. Basis cranii simple. No pharyngeal teeth. Opercular bones hidden under the skin; gill-clefts narrow. Anterior ribs sessile; epineurals, no epipleurals. Pectorals directed upwards. Ventrals with 6 or 7 rays. Air-bladder communicating with the ear.
Fig. 329.—Mormyrus caballus. ⅕ nat. size.
Curious-looking Fishes, very variable in the form of the head and body and in the extent of the fins. Mouth often very small; teeth in jaws usually few; teeth usually present on the parasphenoid, working against a similar patch on the glossohyal; eye covered over by skin, sometimes very indistinct; scales small, cycloid; branchiostegal rays 4 to 8. The dorsal and anal fins may be nearly equally developed (Genyomyrus, Gnathonemus); or the former (Mormyrus) or the latter (Hyperopisus) are several times the longer. Gymnarchus, Eel-shaped, apodal, and deprived of the caudal fin, lacks the anal fin, the dorsal extending over the whole extent of the body. Some species of Mormyrops show how a form like Gymnarchus may have been evolved out of a more typically-formed Fish. Nothing is more striking than the variation in shape of the snout within one and the same genus, and the names given to some of the species (ovis, caballus, elephas, tamandua, numenius, ibis) are suggestive of resemblances with the heads of various animals.
Fig. 330.—Head of Gnathonemus curvirostris.
Fig. 331.—Head of Gnathonemus numenius.
The Mormyrids are highly remarkable for the enormous development of the brain, the weight of which equals 1⁄52 to 1⁄82 of the total, a thing unparalleled among lower Vertebrates; and for the problematic organ which surmounts it; also as being among the few Fishes in which an electric organ has been discovered. The organ, situated on each side of the caudal region, is derived from the muscular system and is of feeble power, as ascertained by Babuchin and by Fritsch; it was long considered as "pseudo-electric." The natural affinities of this family appear to be with the Albulidae, and there is nothing to justify the term "Nilhechte" (Nile-pike) which has been bestowed on them by German authors. Ninety-three species are known from the fresh waters of Africa north of the Tropic of Capricorn, and may be referred to two sub-families and ten genera[[643]]:—