258 DIPNOI 



Ceratodus, Ag. (Fig. 228) ; Triassic and Jurassic, Europe, N. America, 

 Africa, Asia, Australia ; Cretaceous, C. Africa, Patagonia. Ceratochts 

 (Neoceratodus), living in Queensland. Gosfordia, A. S. W. ; Triassic, 

 N.S. Wales. 



Family Lepidosirenidae. The most specialised and degenerate of 

 Dipnoi. The body becomes much elongated, especially in Lepidosiren, 

 the scales are reduced in size, the naked filamentous paired limbs are 

 vestigial (Fig. 228). The dermal fin-rays are soft, scarcely jointed, and 

 with few cells. 



The head is covered with soft, scale-bearing skin, and the dermal 

 bones are deeply sunk, and still further reduced in number. Two large 

 median bones cover the incomplete chondrocranium above ; two elongated 

 lateral bones pass back from the orbits at the sides ; and two ' squamosals ' 

 cover the quadrates (Fig. 209). The dentaries and post-temporals are 

 lost ; the opercular bones are very small. The branchial arches are much 

 reduced, and all trace of the hyomandibular has gone. 



As described above, the teeth, gills, heart, and male genital ducts are 

 highly specialised. The lung-sac is bilobed. A remarkable growth of 

 vascular filaments develops during the breeding season on the pelvic 

 limbs of the male Lepidosiren (Fig. 228). They appear to function as 

 accessory gills (Lankester [278], Kerr [259]). 



Lepidosiren, Nath. ; South America ; Protopterus, Owen ; Tropical 

 Africa. 



Affinities. — It is clear that the early Dipnoi approach closely 

 to the primitive Teleostomes in general structure. Moreover, in 

 both them and the Osteolepiclae we find similar lobate fins, large 

 paired inferior gulars, a layer of typical cosmine, powerful palatine 

 and splenial teeth, and a blunt snout with ventral nostrils. The 

 Dipnoi are probably a specialised offshoot from the base of the 

 Teleostome stem, which acquired an autostylic structure before the 

 hyomandibular had become very large, and before the hyostylism 

 of the ancestor had become fully established. But such a pre- 

 Devonian ancestral form must have differed so considerably from 

 any known genus that it seems better for the present to keep the 

 Dipnoi separate from the Teleostomi in our classification. 



Sub-Class 2. COCCOSTEOMORPHI (Arthrodira). 



A group of heavily armoured Palaeozoic fish, which often 

 attained a formidable size. The large broad head has the orbits 

 placed very far forwards, and apparently two small nasal openings 

 near the extremity of the blunt snout. The pineal eye is indicated 

 by a foramen, or an internal depression, in a median cranial plate. 

 Both the head and the anterior region of the trunk are covered 

 with a shield of closely fitting or fused bony plates. Those on the 

 trunk encircle it in a complete cuirass, which usually articulates 

 with the cranial shield by means of a pair of elaborately differen- 



