﻿INTRODUCTION. Vll 



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 Miiller 1 . Sharks and Rays, with the 

 Uhimaeras, have been generally regarded as an order or subclass, 

 variously termed Elasmobranchii, Chondropteryoii, 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 Polyptents, Acipenser, Lepidosteus, and 

 Amia have been regarded as typifying four groups, to be comprised 

 in an order or subclass Ganoidei ; 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 exo- 

 skeletal ossifications, including a bony gill-cover. The Dipnoi, 

 typified by the existing Lepidosiren, Protopterus, and Ceratodus, 

 have sometimes been included in the Ganoidei, sometimes (as by 

 Miiller) elevated into an equivalent division, on account of their 

 approach to the Amphibia ; while the Teleostei, 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. Gunther 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 Teleostei, 

 and distinguished solely by the three visceral characters already 

 mentioned in connection with the heart, intestine, and optic nerves. 

 To emrjhasize the division all the more clearly, the " Palaeichthyes " 



1 J. Miiller, " Ueber den Bau und die Grenzen der Ganoiden und iiber das 

 natiirliche System der Fiscke," Abh. k. Akad. Wiss., phys. CI., Berlin, 1844, 

 pp. 117-216, with plates. 



2 A. Gunther, Phil. Trans. 1871, p. 554; also ' An Introduction to the Study 

 of Fishes '(1880). 



