XVIII CROSSOPTERYGII 481 
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 inter- 
vening 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' 
(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 endo- 
skeletal 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 primi- 
tive features indicative of their remote origin. The spiracles 
are persistent ; the intestine has a spiral valve; and the conus 
arteriosus 1s 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. 
Fam. 5. Polypteridae.’-— Pectoral fins obtusely lobate. Pelvic 
fins non-lobate. Scales rhombic and thickly enamelled. Dorsal 
fin in the form of a series of isolated finlets, each consisting of a 
stout spine-like* fuleral scale supporting a single soft ray, or a 
fringe of several rays, along its hinder margin. Tail symmetrical, 
apparently gephyrocercal. Teeth simple. Nostrils tubular. 
The only representatives of the sub-order and the sole 
1 Smith Woodward, op. cit. p. 412. 
2 Boulenger, Poiss. Bass. Congo, p. 10. For a list of the more important 
papers, see pp. 18-19 of that work. 
3 Mr. Boulenger informs me that he regards these spines as modified ridge 
scales or fulera. The latter are median spine-like or A-shaped scales in relation 
with the anterior margins of the median fins in some Crossopterygii (e.g. Osteo- 
lepidae) and in many Chondrostei and Holostei. 
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