DIPNOI 



257 



Tribe 2. 



In the following three families the gular plates and the cosmine layer 

 are lost ; the dermal bones sink beneath the skin ; in the living genera 

 the lateral-line system is in the skin, the organs being quite superficial 

 on the body, and sunk in canals only on certain regions of the head ; the 

 tail is diphycercal, and the median fins are continuous. They show- 

 progressive specialisation and degeneration, though probably not forming 

 a true monophyletic series. 



Family Ctenodontidae. The covering bones of the skull resemble 

 those of Dipterus ; but the frontals and parietals are of considerable size, 



Fig. 228. 



A, Ceratodus Forsteri, Krefffc. B, Protopterusannectens, Owen (after Lankester). C, Lepidosiren 

 paradoxa, Fitz. (after Lankester, modified), b.o, branchial opening ; /, median tin ; p.f, pectoral 

 tin ; pv, pelvic fin ; vf, vascular villi present on the male. 



and the anterior median plate is small (Fig. 209). The ridges of the 

 teeth are set with tubercles. 



Ctenodus, Ag. ; Carboniferous, Europe and X. America. Sagenodus, 

 Owen ; Carboniferous, Europe. 



Family Ceratodidae. The cranial bones are thin and much reduced 

 in number. Two large median bones, 'ethmoid' and ' occipital ' (p. 238), 

 and two large paired ' lateral ' bones, probably including the frontals 

 and parietals, cover the head, and are themselves overlaid with scales. 

 Circumorbital bones, postfrontals and ' squamosals,' complete the covering. 

 The chondrocranium is thick, complete, and unossified (Figs. 206-7). 

 The hyomandibular persists as a vestige ; the hyoid and branchial arches 

 are better developed than in the Lepidosirenidae. The gills are well 

 developed, and the air-bladder or lung is a median sac. 



The paired fins have an endoskeleton with biserial radials, and are 

 covered with scales (Fig. 213). The ridges on the teeth have lost the' 

 tubercles. 



17 



