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 1 

 (Fig. 278) is a Jurassic genus. Diplurus is found in the Trias 

 of North America, and Jdacropoma 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 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. 



Fam. 5. Polypteridae. 2 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 3 fulcral 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 fulcra. 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. 



VOL. VII 2 I 



