838 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 



forms 1 , and how far recent researches enable us to improve upon 

 Muller's definitions. In his classical memoir (No. 13) the charac- 

 ters of the Ganoids are thus shortly stated : 



" These Fishes are either provided with plate-like angular or 

 rounded cement-covered scales, or they bear osseous plates, or 

 are quite naked. The fins are often, but not always, beset with 

 a double or single row of spinous plates or splints. The caudal 

 fin occasionally embraces in its upper lobe the end of the ver- 

 tebral column, which may be prolonged to the end of the upper 

 lobe. Their double nasal openings resemble those of Teleostei. 

 The gills are free, and lie in a branchial cavity under an oper- 

 culum, like those of Teleostei. Many of them have an accessory 

 organ of respiration, in the form of an opercular gill, which is 

 distinct from the pseudobranch, and can be present together 

 with the latter ; many also have spiracles like Elasmobranchii. 

 They have many valves in the stem of the aorta like the latter, 

 also a muscular coat in the stem of the aorta. Their ova are 

 transported from the abdominal cavity by oviducts. Their optic 

 nerves do not cross each other. The intestine is often provided 

 with a spiral valve, like Elasmobranchii. They have a swim- 

 ming-bladder with a duct, like many Teleostei. Their pelvic 

 fins are abdominal. 



" If we include in a definition only those characters which 

 are invariable, the Ganoids may be shortly defined as being 

 those Fish with numerous valves to the stem of the aorta, which 

 is also provided with a muscular coat ; with free gills and an 

 operculum, and with abdominal pelvic fins." 



To these distinctive characters, he adds in an appendix to 

 his paper, the presence of the spiral valve, and the absence of a 

 processus falciformis and a choroid gland. 



To the distinctive set of characters given by M tiller we may 

 probably add the following : 



(1) Oviducts and urinary ducts always unite, and open by a 

 common urinogenital aperture behind the anus. 



(2) Skull hyostylic. 



1 We do not profess to be able to discuss this question for extinct forms of Fish, 

 though of course it is a necessary consequence of the theory of descent that the various 

 groups should merge into each other as we go back in geological time. 



