INTRODUCTION. 7 



discharge of one into the other. Usually the male and female ducts open into the 

 dilated ureters ; in Polypterus the united ureters open into the united generative ducts. 

 In Sirenoidei the ureters, separate or united, and the united generative ducts are 

 received into an urogenital sinus which lies in front of the rectum, and opens in common 

 with it. Abdominal pores, single or double, situate behind the anus, occur in all 

 Ganoids. 



It will be seen that the urogenital system furnishes characters by which the Ganoids 

 are removed from Teleostei, and approximated to Elasmobranchii, but that the corre- 

 spondence with the latter group is far from close. 



Some of the less constant features of the Ganoids are available for subdivision of the 

 order into groups adequate to the purposes of the zoologist if not of the palaeontologist. 

 Amia, Polypterus, and Lepidosteus are covered with scales and have a bony skeleton ; 

 the Sturgeons, Scaphirhynchus, and Polyodon are naked or only partially covered with 

 osseous scutes, while their endo-skeleton is incompletely ossified, especially in the skull. 

 These points of difference define two sections — the Holostei and the Chondrostei of 

 Muller. We may employ for generic distinction the differences in the respiratory 

 organs, already given in a tabular form, and the varying structure of scales and fins. 

 Amiais cycloid; Polypterus and Lepidosteus ganoid in the sense of Agassiz. Lepidosteus 

 and the Chondrostei have fulcra or imbricate, pointed scales attached generally to the 

 front edge of some of the fins ; Amia, Polypterus, and Calamoichthys have fins without 

 fulcra. Most of the Ganoids have abdominal ventral fins ; Calamoichthys has no ventral 

 fins at all. 



Muller's discoveries were sufficient to define the Ganoid order, to subdivide it into 

 two convenient sections, and to arrange under these with just and precise definitions 

 all the then known recent genera. It now remains to narrate briefly the progress of 

 discovery since 1844, and to discuss the position of the Ganoids in the class Pisces as 

 disclosed to us by further investigations. 



The discovery of Lepidosiren and Prolopterus, and the foundation thereupon by 

 Muller of the order Dipnoi, are referred to at greater length under the genus Lepidosiren. 

 This order was recognised as perfectly distinct until the announcement in 1870 of one 

 or more living species of Ceratodus, and the revelation of its structure by Dr. Gunther. 

 The influence of this discovery upon systematic ichthyology must be shortly noticed in 

 this place. Nearly all the characters of Lepidosiren and Protopterus assimilate them to 

 the class Pisces. In that class evidence of their special relationship to the Ganoids, 

 Sharks, and Rays, was afforded by their muscular and contractile bulbus arteriosus, the 

 valves in its interior, and the spiral valve of the intestine. Owen, Hyrtl, and Huxley 

 had remarked the points of agreement between Lepidosiren and the Ganoid Polypterus. 

 The two genera were indeed referred to distinct orders of fishes mainly on account of the 

 different function of the swim-bladder and the different position of the valves of the 



