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Encyclopedia Britannica



ORDER I

This article appears in Volume V26, Page 543 of the Encyclopedia Britannica.

Encyclopedia Britannica - Main :: ORC-PAI
ORDER I .CROSSOPTERYGII
Paired fins, at least the pectorals, lobate, having an endo-skeletal
axis
  more or less fringed with dermal rays. Mandibular arch suspended from the upper segment of the hyoid arch (hyostylic skull). Splenial bone present. No supraoccipital bone. A pair of large jugular plates, sometimes with small lateral plates and an anterior azygous
element
 , developed in the branchiostegal membrane between the mandibular rami. Heart with a' contractile, multivalvular conus arteriosus; intestine with a spiral valve; air-bladder with pneumatic duct communicating with the ventral side of the oesophagus.
Maxillary bone large, toothed, bordering the mouth. Bones of the upper surface of the skull mostly paired. Pectoral arch with both clavicle (so-called infra-clavicle) and cleithrum. Ventral fins inserted far back. With few exceptions (tail of Coelacanthidae, dorsal and caudal fins of Polypteridae) the dermal rays of the unpaired fins more numerous than their endo-skeletal supports, a primitive character also found in the lower Ganoids, but disappearing in the higher.
SUB-ORDER I.OSTEOLEPIDA
(Including the Haplistia, Rhipidistia and Actinistia.) Pectoral fins obtusely or acutely lobate, articulating with the pectoral
girdle
  by a single basal endo-skeletal
element
 . Nostrils on the lower side of the snout. Two dorsal fins.
Families: Osteolepidae, Rhizodontidae, Holoptychidae, Coelacanthidae.
The scales may be rhombic and thickly coated with ganoine (Osteolepidae) or cycloid. The vertebral
axis
  is strongly heterocercal in the Osteolepidae and Holoptychidae, and diphycercal or intermediate between the heterocercal and the diphycercal types in the other families; usually acentrous, sometimes with ring-like calcifications (some of the Rhizodontidae). In the Holoptychidae the pectoral fin is extremely similar to that of the Dipneusti of the family Dipteridae, which they resemble closely in form and scaling. Their teeth are remarkable for their complicated structure, resembling that of the Labyrinthodont Batrachians. A pineal foramen is present between the frontal bones in most of the Rhizodontidae.
The Osteolepidae were mostly moderate-sized fishes, the largest (Megalichthys) measuring about 4 ft. in length.
These Crossopterygians first appear in the Lower Devonian, are abundant in the Upper Devonian, Carboniferous and Permian; in later periods they are represented only by the more specialized Coelacanthidae, which appear in the Lower Carboniferous, and persist as
late
  as the Upper Chalk.
SUB-ORDER II.CLADISTIA
Pectoral fin obtusely lobate, with three basal endo-skeletal elements. Nostrils on the upper side of the snout. A single dorsal fin, formed of a
series
  of detached rays.
A single family: Polypteridae.
The existing Crossopterygians which form this sub-order differ very considerably from the
extinct
  Osteolepida, perhaps quite as much as these differ from the Dipneusti. The ventral fins are not lobate, the vertebral column is well ossified and its termination is of the diphycercal type. Spiracles, covered by bony valves, are present on the upper surface of the head. The dorsal fin is unique among fishes, being formed of detached rays consisting of a spine-like fulcra! scale supporting the fringes of the ray; these rays have been regarded, erroneously, as representing so many distinct fins, or " finlets." The scales are bony, rhombic and thickly coated with ganoine.
The Polypteridae are confined to tropical Africa and the Nile, and represented by two genera: Polypterus and Calamichthys, the former moderately elongate and provided with ventral fins, the
latter serpentiform and devoid of ventrals. We now know ten species of Polypterus, from the Nile, the Congo, the rivers of West Africa, and lakes Chad, Rudolf and Tanganyika, and one of Calamichthys, which inhabits West Africa from the Niger
delta
  to the Chiloango. The largest species of Polypterus reach a length of nearly 4 ft. The
young
  are provided with an external opercular gill very similar to the gills of larval salamanders. The air-bladder acts as an accessory breathing organ, although these fishes are not known ever to leave the water. The development is stated by the
late
  J. S. Budgett to be even more Batrachian-like than that of the Dipneusti, but the results of the study of the material collected by him shortly before his death have not yet been published.


End of Article: ORDER I


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