STRUCTURE AND DEVELOPMENT OE LEPIDOSTEUS. 
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(8.) The absence of a pecten, and presence of a vascular membrane between the 
vitreous humour and the retina. 
(9.) The opisthoccelous form of the vertebrae. 
(10.) The articulation of the ventral parts of the haemal arches of the tail with 
processes of the vertebral column. 
(11.) The absence of a division of the muscles into dorso-lateral and ventro-lateral 
divisions. 
(12.) The complete segmentation of the ovum. 
The list just given appears to us sufficient to demonstrate that Lepidosteus cannot 
be classed with the Teleostei; and we hold that Muller’s view is correct, according 
to which Lepidosteus is a true Ganoid. 
The existence of the Ganoids as a distinct group has, however, recently been 
challenged by so distinguished an Ichthyologist as Gunther, 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 improve upon Muller’s 
definitions. In his classical memoir (No. 13) 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 
occasionally embraces 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 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 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. 
* 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. 
MDCCCLXXXTI. 3 K 
