VERTEBRATA: FISHES. 



503 



of a single auricle and ventricle, with a " Ganoid " bulbus 

 arteriosus. There are five branchial arches, of the Teleostean 

 type, but cartilaginous. The swim-bladder is single, composed 

 of two symmetrical halves, cellular in structure, with a -pneu- 

 matic duct and glottis, as in Lepidosiren. The intestine has a 

 spiral valve, and there are no pyloric caeca. There are two 

 molar teeth in each jaw, having the form of flattened undu- 

 lated plates of bone, singularly like the teeth of Ceratodus 

 from the Trias (fig. 274). The Ceratodi employ these crush- 



Fig. 274. A, Dental plate of Ceratodus serratiis, Keuper. B, Dental plate of 

 Ceratodus altus, Keuper. (After Agassiz.) 



ing teeth in the mastication of vegetable matter, upon which 

 they feed ; and they are stated to leave the streams which 

 they inhabit, at night time, in order to betake themselves to 

 the marshy flats in the vicinity, where they obtain an abun- 

 dant supply of food. 



The genera Lepidosiren (with Protopterus) and Ceratodus 

 comprise the few living species of Dipnoi, and constitute a 

 special division (Sirenoidei) characterised by the possession of 

 horny cycloidal scales, a symmetrically divided caudal fin, and 

 an undivided dorsal fin. There are, however, a few Palaeozoic 

 genera of Dipnoi of which the most important are Dipterus 

 and Ctenodus which constitute a distinct sub-order (Ctenodip- 

 terini), characterised by their enamelled scales, their hetero- 

 cercal tails, and their possession of two dorsal fins. 



Upon the whole, Dr Giinther concludes that the Dipnoi are 

 to be regarded as a simple sub-order of Ganoids, and that the 

 entire order Ganoidei may be united with Elasmobranchii 

 into a single order, called Palaichthyts> characterised by having 

 a " heart, with a contractile bulbus arteriosus, intestine, with a 

 spiral valve, and optic nerves non-decussating." 



