952 CLASS PISCES. 



by the presence of a remarkable spine-like tooth placed in front of 

 the normal tooth of the mandible. The fin-spines of this genus 

 approximate to those of the Elasmobranchian genus Aster acanthus. 

 Rhynchodes from the Devonian of Ohio, and Ptyctodus from that of 

 both Russia and Illinois, are genera founded upon teeth which their 

 describers refer to this order. Some of the genera founded upon 

 the evidence of detached fin-spines, a few of which are noticed in 

 the preceding chapter, should perhaps also find a place among the 

 Chimaeroids. m 



Finally, the genus Coelorhynchus may be mentioned in this con- 

 nection. It was founded upon spines originally regarded as the 

 rostra of Sword-fishes, but which are evidently of dermal origin, and 

 are considered by Mr S. Woodward as being probably fin-spines of 

 Chimaeroids. These spines occur in the Chalk of England and 

 Maastricht, in the Lower Eocene of England, Egypt, and India, and 

 also in the Middle Eocene of Bracklesham in Sussex. 



Order IV. Dipnoi. — The Dipnoi, which Dr Giinther regards 

 as a subdivision of the Ganoidei, are typically freshwater fishes, 

 usually presenting the following characters : The body is covered 

 with imbricating cycloidal scales, while the vertebral column is car- 

 tilaginous, and there are both anterior and posterior nostrils placed 

 more or less within the mouth (fig. 88 1). The primitive cartila- 

 ginous cranium persists more or less completely, and, like that 

 of the Chimaeroids, is autostylic ; cranial membrane bones are, 

 however, always developed to a certain extent, and there are also 

 splenial and articular bones in the mandible, while the cranium is 

 immovably connected with the vertebral column. The palatopteryg- 

 oid bar persists as the functional upper jaw, and, as in the living 

 Chimaeroids, carries a single pair of molariform teeth, while a cor- 

 responding pair of teeth are placed on the splenial and articular 

 bones of the mandible. There is also a smaller pair of vomerine 

 teeth ; in advance of which there may be other minute teeth. The 

 paired fins have a long, cartilaginous, jointed, median axis (fig. 845), 

 and the tail may be either diphycercal or heterocercal. There are 

 no functional branchiostegal rays ; and the five or six cartilaginous 

 branchial arches are more or less rudimentary, and their single aper- 

 ture is closed by a gill-cover. The teeth agree with those of the 

 Chimaeroids in having no successors ; but, from the presence of 

 membrane bones, do not constitute the whole of the jaw. 



In the structure of their skull the Dipnoi show affinities to Chi- 

 maeroids, Ganoids, Teleosteans, and Amphibians ; the autostylic 

 feature connecting them with the former, and the double nares 

 with the latter, in which the skull is also autostylic. The lungs are 

 formed by the connection of the swim-bladder with the gullet by 

 means of a duct, and these fishes can thus either breathe by means 



