﻿Vlll INTRODUCTION. 



are arranged in ascending series, so far as can be determined., while 

 the " Teleostei " are treated in precisely the opposite order. 



This dual subdivision may appear, at first sight, to be the logical 

 result of Agassiz's recognition of the primitive nature of the typical 

 ft Ganoidei," — especially when added to Miiller's subsequent discovery 

 of the important characters these fishes possess in common with the 

 Elasmobranchs, Chimaeroids, and Dipnoans. A consideration of the 

 researches of Agassiz himself, however, suffices to demonstrate that 

 if gradations in skeletal anatomy are more or less concomitant, as 

 usual, with the evolution of the soft parts, every essential link be- 

 tween the " Ganoidei " and " Teleostei " is already known. So long 

 ago as 1866, this fact was clearly recognized by Owen l , when he 

 proposed to group the Ganoids and Teleosteans in a subclass Teleos- 

 tomi, adopting the Plagiostomi (including Holocephali) and Dipnoi 

 as equivalent divisions. About the same time, Kner 2 concluded that 

 the group of Ganoidei was not homogeneous, and was, at least in part, 

 separated too widely from the Teleosteans by Agassiz. The sub- 

 sequent investigations of Cope 3 , Gill 4 , Liitken 5 , and Huxley 6 have 

 tended in the same direction ; and the most recent statements of 

 the last-named author concerning points of visceral anatomy will be 

 generally regarded as final and conclusive. The researches of Boas 

 are cited to prove that there is no absolute distinction between 

 Ganoids and Teleosteans in the conus arteriosus of the heart; 

 the rudiment of a spiral valve in the intestine of Ohirocentrus is 

 noticed as rendering a second point of the original Mullerian 

 diagnosis invalid ; while a reference to Wiedersheim's discovery 

 of the partial decussation of the fibres of the optic nerves 

 in some lizards, suggests that if this feature be of little syste- 

 matic importance in an order of Reptiles, it is not likely to be 



1 E. Owen, ' Anatomy of Vertebrates,' vol. i. (1866), p. 7. 



2 R. Kner, Sitzungsb. k. Akad. Wiss., math.-naturw. CI. vol. liv. pt. i. (1866), 

 pp. 519-536. 



3 E. D. Cope, Trans. Amer. Phil. Soc, n. s., vol. xiv. (1871), pp. 445-460 ; 

 Proc. Amer. Assoc. Adv. Sci. 1871 (1872), pp. 317-343 ; Amer. Nat. vol. xix. 

 (1885), pp. 234-243; ibid. vol. xx. (1886), p. 1031; ibid. vol. xxi. (1887), 

 pp. 1014-1019; ibid. vol. xxiii. (1889), pp. 852-860; Proc. Amer. Phil. Soc. 

 1884, pp. 577-585. 



4 T. Gill, Proc. Acad. Nat. Sci. Philad. 1861, pp. 12-20 ; and " Arrangement 

 of the Families of Fishes," Smithsonian Miscell. Coll. vol. xi. (1872). 



5 C. Liitken, " TJeber die Begrenzung und Eintheilung der Ganoiden," Palse- 

 ontographica, vol. xxii. (1873), p. 1 ; translated from Yidenskab. Meddel. Naturh. 

 Foren. Kjobenhavn, 1868. 



6 See especially Proc. Zool. Soc. 1876, pp. 24-59, and ibid. 1883, pp. 137-139. 



