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 CTEXODONTIDAE. The covering bones of the skull resemble 

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



vf 



FIG. 228. 



A, Cerutodus Fm-xtei-i, Krrtt't. B, Protopterusannectent:, Owen (after Lankester). C, Lepidosiren 

 iHtrmiuxa, Fitz. (after Lankester, modified). 6.0, branchial opening ; /, median tin ; p.f, pectoral 

 fin ; pv, pelvic fin ; vf, va.scular 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 N. 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 



