Fig. 276.—Restoration of Eusthenopteron foordi. Upper Devonian of Scaumenac Bay, Canada. The scales have been omitted in the hinder part of the body to show the vertebral column and the radials of the median fins. cl, Clavicle; i.cl, infra-clavicle; s.cl, supra-clavicle; for other reference letters see Fig. 274. × about ¼. (After Whiteaves.)
Fig. 277.—Restoration of Holoptychius flemingi. × ⅛. (From Traquair.)
Fam. 3. Holoptychidae (Dendrodontidae).—Scales cycloid. Pectoral fins acutely lobate; pelvic fins short and somewhat obtusely lobate. Tail heterocercal. Teeth similar to those of the Rhizodontidae but more specialised, the enamelled dentine infoldings being much more complicated, presenting a radiating arborescent appearance in transverse sections. Vertebral column acentrous. Genera:—Holoptychius[[565]] (Fig. 277), Old Red Sandstone of Scotland; Devonian of Belgium, Russia, North America, and East Greenland. Glyptolepis has a similar range.
Fig. 278.—Restoration of Undina gulo. Lower Lias of Dorset. Scales and supra-clavicle omitted. The ossified air-bladder is shown beneath the anterior part of the vertebral column. The facial bones in front of the orbit are unknown, and the cheek-plates are supposed to be arranged as in other Coelacanths. × about ⅐. (From Smith Woodward.)
Fam. 4. Coelacanthidae.[[566]]—Scales cycloid. Paired fins obtusely lobate. Tail symmetrical but apparently gephyrocercal, usually with a protruding axial vestige of the disappearing terminal part of the tail and of the proper caudal fin. Radialia of the functional caudal lobes agree in number with the contiguous neural and haemal arches and dermal fin-rays, the diagnostic feature of Smith Woodward's Actinistia. Proximal radials of the dorsal and anal fins fused into a single, internally-forked basipterygium in each fin. Teeth simple. Vertebral column acentrous. The skull presents several interesting features. The hyomandibular and the palato-quadrate bar, for example, are fused on each side into a continuous triangular bone, articulating with the cranium above and with the lower jaw below. The opercular skeleton is reduced to an operculum and two jugular plates. A very singular feature in these Fishes is the ossification of the walls of the air-bladder (Fig. 278), a structural modification which has no parallel in Fishes, except in certain Teleosts (Siluridae and Cyprinidae)[[567]] in which the organ becomes encapsuled by bone owing to the partial ossification of its walls.
From their first appearance in the Lower Carboniferous the Coelacanthidae range, practically unchanged, through the intervening formations to the Upper Cretaceous. Coelacanthus itself occurs in the Carboniferous and Permian of England, Scotland, and Germany, and in the Carboniferous of North America. Undina[[568]] (Fig. 278) is a Jurassic genus. Diplurus is found in the Trias of North America, and Macropoma is a well-known form from the Middle and Upper Cretaceous beds of England, and other parts of Europe.
Sub-Order 2. Cladistia.
Pectoral fins uniserial and abbreviate, with three basal endoskeletal elements. Nostrils on the upper surface of the snout. Entire skeleton well ossified. Notochord replaced by bony, amphicoelous vertebral centra. Bones of the ethmoid region not fused to form a rostral shield. Infra-dentary bones absent. Jugular plates reduced to a single pair of large plates. As this group includes the only Crossopterygii which have survived to the present day, it is noteworthy that they retain certain primitive features indicative of their remote origin. The spiracles are persistent; the intestine has a spiral valve; and the conus arteriosus is furnished with several rows of valves. Amongst other characters of contrary significance, the air-bladder is double; its oesophageal aperture is ventral; and its afferent arteries are pulmonary arteries derived from a posterior aortic arch.