IXTR0DT7C . ION. 
Yll 
merely an instance of parallel development in the different groups ; 
the same laws prevailing in each great division and producing 
analogous results. 
Such being the case, the difficult question arises as to what cha¬ 
racters determine the Subclasses (as we prefer to term the great 
divisions), according to the most recent researches. For a long 
period, as is well known, it has been a prevalent custom, at least in 
Europe, to follow the combined arrangements of Cuvier and Agassiz 
as modified by Johannes Muller 1 . Sharks and Rays, with the 
Chimaeras, have been generally regarded as an order or subclass, 
variously termed Elasmobraxchii, Choxdropterygii, Selachii, or 
Placoidei, and specially characterized (i) by the absence of mem¬ 
brane-bones or true ossifications of any kind, (ii) by the arrangement 
of the gills, and (iii) by the characters of the brain, heart, intestine, 
and ovaries. The recent Polypterus , Acipenser , Lepidosteus , and 
Amia have been regarded as typifying four groups, to be comprised 
in an order or subclass Gaxoidei : this agreeing with the Elasmo- 
branchii in the more important visceral characters, though distin¬ 
guished by the presence of an air-bladder, the small size of the 
numerous ova, and the development of both endoskeletal and ex^ 
skeletal ossifications, including a bony gill-cover. The Dipxoi, 
typified by the existing Lepidosiren , Protopterus, and Ceratodus , 
have sometimes been included in the Ganoidei, sometimes (as by 
Muller) elevated into an equivalent division, on account of their 
approach to the Amphibia ; while the Teleostet, or modern bony 
fishes, with decussating optic nerves, no intestinal spiral valve, and a 
non-contractile bulbus arteriosus to the heart, have constituted the 
highest order or subclass, specially characteristic of the existing fauna. 
Dr. Giinther 2 proceeds further than all the other authors in 
elaborating this scheme of classification, uniting the Elasmobranchii 
and Ganoidei (including the Dipnoi) in a great subclass of 
Paljeichthyes ; this to be equivalent in value to the Teleostet, 
and distinguished solely by the three visceral characters already 
mentioned in connection with the heart, intestine, and optic nerves. 
To emphasize the division all the more clearly, the “ Palaeichthyes 
1 J. Miiller, “ Ueber den Bau und die Grenzen der Ganoiden und liber das 
natiirliche System der Fische," Abb. k. Akad. Wiss., pbys. Cl., Berlin, 1844, 
pp. 117-216, with plates. 
2 A. Gunther, Phil. Trans. 1871, p. 554; also/ An Introduction to the Study 
of Fishes’ (1880). 
