POLYPTERUS, a genus of Ganoid fishes common in many rivers of tropical Africa, and known on the Nile by the name of abü bishïr. Their body is cylindrical in shape, elongate, and covered with hard, polished, ganoid scales, which are arranged in oblique series. The head, with flattened snout and wide mouth, is protected by bony plates with ganoid external surface, of which a series of “supra-temporal” and “spiracular” ossicles are especially characteristic. Spiracles, or external openings of a canal leading into the pharynx are persistent throughout life, situated on each side of the parietal bone, and closed by an osseous valve. The lips are fleshy, but the space between the rami of the mandible is covered by a large “gular” plate. The vent is placed far backwards, in front of the anal fin, the tail being short, with a diphycercal termination of the vertebral column. The mouth is well provided with rasp-like teeth, forming broad bands in the jaws, on the vomer and palatine bones. The paired fins are supported by an axial skeleton. The structure of the dorsal fin is unique: its anterior portion is composed of isolated finlets, from eight to eighteen in number, each of which consists of a flattened spine with a bifurcate termination; to the posterior aspect of the top of each spine several soft rays are attached, which result from the dichotomous division of a single ray, the basal portion of which is the spine. Posteriorly these finlets pass into the ordinary rays composing the caudal fin, which surrounds the tail. The ventral fins are well-developed, and inserted behind the middle of the length of the trunk. The respiratory apparatus consists of three and a half gills, and is protected by an osseous gill-cover....
An external gill of considerable size in the form of a tapering band fringed with respiratory laminae exists in young examples, and is attached to the end of the gillcover. The air-bladder is double, and communicates with the ventral wall of the pharynx.
Such are some of the principal characteristics of one of the most interesting representatives of a type which in Polypterus has survived from the Devonian and Carboniferous formations to our period; for further details of its internal organization see Ichthyology. The centre of distribution of Polypterus is the lake region of tropical Africa, from which the Nile and the great ri∖ers of West Africa take their origin. A very remarkable fact is its total absence in the East-African river systems which belong to tl.e Indian Ocean. Specimens of the bishir have been found in the Nile as low as Cairo, but it is very scarce throughout the middle and lower parts of that river; such individuals have evidently been carried by the current down from southern latitudes, and do not propagate the species in the northern parts. As mentioned above, the number of the rays which are modified into finlets varies considerably, and consequently several species have been distinguished by some naturalists, whilst others hold that the∣e is one species of Polypterus only. The largest specimens observed had a length of 4 feet. Nothing is known of its habits and propagation, and observations thereon are very desirable. Some years ago an extremely interesting dwarf form of Polypterus was discovered in Old Calabar, and described under the name of Calav ∣ oichthys calabaricus. It much resembles the bishir but is smaller, and considerably more elongate.
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