Dip no i Sirenoidei. 



95 



nostrils more or less within the month (Fig. 131, n) ; and the 

 dentition (Fig. 131) consists of a pair of ridged plates above 

 and below, usually with a pair of incisor-like vomerine teeth 

 above. An ordinary bony operculum covers the gill-cavity. 

 The paired fins are acutely lobate, supported by a central 

 jointed cartilaginous stem fringed with radial cartilages and 

 dermal fin-rays. 



The earliest Dipnoi are Lower Devonian. Dipterus (Fig. Wall-case, 

 132) is beautifully preserved in the Caithness flagstones, ^ablt-oSe 

 and exhibits two dorsal fins, a heterocercal tail, and enamelled ]^ . 34, 

 scales. Phaneropleuron (Fig. 133) occurs in the Upper Old Bed 

 Sandstone of Dura Den, Fifeshire, and Scaumenacia in the 

 Upper Devonian of Canada. Ctenodus is characteristic of the 

 Carboniferous and Lower Permian, and is met with both in 

 Europe and America. All these genera are characterized by 

 the roof of the skull exhibiting more numerous bones than 

 that of the living Dipnoi. The teeth of Ceratodus occur in 

 early Mesozoic strata in Europe, India, South Africa, and 

 Central North America ; but only one important skull has 



FIG. 131. Mouth of Ceratodus. nn, narial openings ; x, vomerine teeth; xx, palato-pterygoid 

 teeth ; xxx, manclibular teeth. 



been described, this from the RliEetic of Austria. A fine series 

 of teeth is exhibited from the Rhsetic of Aust Cliff near 



FIG. 132. Dipterus Valenciennesi, Sedgw. and Murch. (restored by C. H. Pander); Lower Old 

 Red Sandstone, Scotland. 



