56 EISH GALLERY. 



similar to those of the Chimaeras, namely a pair of mandibular plates 

 below, and a pair of large palatine plates and a pair of small 

 vomerines above. They are tuberculated, as though made up of 

 fused denticles. 



The notochord is persistent, with unsegmented sheath and 

 without vertebral centra. The vertebral axis of the tail is uptilted 

 in most of the ancient forms, but the tail has a straight axis and 

 a tapering, symmetrical outline in the recent forms. The paired 

 fins are long and pointed, and each has a central, muscular, scale- 

 covered lobe, and a fringe or marginal membrane supported by 

 closely-set dermal fin-rays. 



The gills are covered by a movable operculum or gill-cover, 

 devoid of branchiostegal rays. The nasal sacs open into the mouth 

 • (fig. 32, p. 55) as well as on to the exterior of the snout, a condition 



met with in Amphibia and higher Vertebrates, but very uncommon 

 in fishes. The conus arteriosus of the heart is spirally twisted, 

 and is provided with several longitudinal rows of valves. The other 

 chambers of the heart are partially divided into right and left parts, 

 the left part carrying the blood from the air-bladder or lung-sac. 

 In the intestine is a spiral valve, and the intestine and the urinary 

 and genital ducts open into a common cloaca. The roof of the 

 mid-brain is not divided into right and left optic lobes, and 

 the optic nerves meet below the brain in the form of a cross or 

 " chiasma." 



CTENODIPTERINI. 



The Ctenodipterini are extinct Dipnoi of the Devonian, 

 Carboniferous, and Permian epochs. The skull has numerous roof- 

 bones; the bones of the gill-cover are less reduced, and the body- 

 scales are thicker than in the living Dipnoi. The principal families 

 are the Ctenodontidse, Dipteridse, and Phaneropleuridse, represented 

 respectively in Wall-case 6 by a tooth-plate of Ctenodus (170), 

 Dipterus. and restored models of the complete fish of Dipterus (168, and 

 fig. 33) and Phaneropleuron (169, and rig. 33). In the Dipterida; 

 the vertebral axis of the tail is uptilted (heterocercal tail), while in 



