Fig. 281.—Larva of Polypterus senegalus. × 4. Showing its characteristic attitude when resting on the bottom of an aquarium, and the large size of the cutaneous gills. (From Budgett.)
In the genus Calamichthys the body is greatly elongate and Eel-like in shape. Pelvic fins are absent, and normally there is no suboperculum. The dorsal finlets are more isolated than in Polypterus, and each spine supports but a single soft ray. Only a single species is known, C. calabaricus[[577]] (Fig. 282).
Fig. 282.—Calamichthys calabaricus. × ⅔. (From a specimen in the Cambridge University Museum.)
Calamichthys has a more restricted distribution than Polypterus, and is confined to certain rivers of West Africa. First obtained at Creek Town on the Old Calabar river, it is now known to occur in the delta of the Niger, on the coast of Cameroon, and as far south as the river Chiloango, frequenting the smaller muddy rivers opening into the estuaries.[[578]] It is a very agile Fish, swimming like a snake, and subsisting on insects and crustaceans. The anal fin is enlarged in the male, and the young are provided with cutaneous gills. Calamichthys may attain a length of nearly 40 cm.
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In the remaining Teleostomi (Actinopterygii) the paired fins are invariably non-lobate, with abbreviate, multibasal endoskeletal supports. Fin-rays are the main support of both the median and paired fins. Jugular plates are usually replaced by branchiostegal rays, but both may co-exist. The Actinopterygii are the successors of the Crossopterygii in palaeontological sequence, and when the latter began to decline in Carboniferous and Permian times, the former, mainly represented by the earlier Chondrostei, had already become the dominant Fishes of the period.
Order II. Chondrostei (Acipenseroidei).
In these Fishes, the oldest and the most primitive of the Actinopterygii, the fin-rays of the median fins still continue to retain their primitive numerical superiority over the radials, and the tail is heterocercal. There is a single dorsal and an anal fin, which, like the upper lobe of the caudal fin, are generally provided with fulcra. Pelvic fins abdominal. Squamation typically rhombic and ganoid. Vertebral column acentrous. So far as is known the chondrocranium is but little ossified, and the cranial bones are mainly dermal. The secondary pectoral girdle still includes a pair of infra-clavicles.