The Ganoids 7 



ologist as Giinther, and it may therefore be well to consider 

 how far the group as defined by Muller is a natural one for 

 living forms, and how far recent researches enable us to im- 

 prove upon Muller 's definitions. In his classical memoir the 

 characters 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 embraces occasionally in its upper lobe the end of the 

 vertebral 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 operculum, 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 Elas- 

 mobranchii. 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 ovi- 

 ducts. Their optic nerves do not cross each other. The in- 

 testine is often provided with a spiral valve, like Elasmobranchii. 

 They have a swimming-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 Muller we 

 may probably add the following: 



" (i) Oviducts and urinary ducts always unite, and open 

 by a common urogenital aperture behind the anus. 



" (2) Skull hyostylic. 



" (3) Segmentation complete in the types so far investigated, 



