ii8 PISCES 



the armour-bearing Teleosteans. Huxley [227-8], Cope [91], Zittel 

 [512], and others have since made important contributions. To 

 A. Giinther we are indebted for many improvements, especially in 

 the detailed classification of modern fish [189, 191]. His grouping 

 of the Pisces in the two sub-classes Palaeichthyes and Teleostei 

 does not express a phylogenetic division. The Palaeichthyes are 

 characterised by the possession of an optic chiasma, a contractile 

 conus arteriosus, and a spiral intestinal valve, as was shown by 

 J. Miiller, C. Vogt, and others. Now these are all primitive 

 ancestral characters in the class. They appear to have been lost 

 by the Teleostei, not to have been acquired by a diverging single 

 branch giving rise to the Palaeichthyes. 



Modern advances in the taxonomy of fish are chiefly due to 

 Traquair [441-469], Smith Woodward [505], Gill [164-5], Boulenger 

 [40-42], and others. 



Following, rather, the example of Cuvier, Valenciennes, and 

 Dum6ril [124], we divide the Pisces into two main groups, corre- 

 sponding to two diverging sub -classes, the Elasmobranchii and the 

 Teleostomi (see Table of Contents, p. vii). Undoubtedly the Dipnoi 

 either are a specialised branch of the Teleostomi or have, at all 

 events, been derived with these from a common ancestral stock. 

 The sub-classes Dipnoi and Teleostomi, therefore, form one division, 

 the Osteichthyes. With the Elasmobranchii are associated two 

 extinct sub-classes (Pleuracanthodii and Cladoselachii) ; together 

 these make up the second great division, the Chondrichthyes. 

 Between these two divisions may provisionally be placed an 

 assemblage of extinct fish, the Ostracodermi, whose affinities are 

 very imperfectly known. Another group of doubtful origin, the 

 Acanthodii, is provisionally retained with the Chondrichthyes 

 (Stannius [417], Huxley [229], Bridge [57], Brown Goode [64], 

 Jordan [250], Gregory [184], Hay [204]). 



Sub-Grade I. CHONDRICHTHYES. 



This is a provisional assemblage of fish which, speaking 

 generally, are lowly organised, and preserve several primitive 

 features. The normal optic chiasma, the contractile conus 

 arteriosus, and the spiral intestinal valve are all present in the 

 living forms. 



True bone is never developed, either in connection with the 

 endoskeleton or in the form of superficial plates and scales. The 

 dermal exoskeleton is entirely composed of ' placoid ' scales or 

 denticles (except for the dermal fin-rays, p. 122, and Acanthodii). 

 Very rarely (Acanthodii, p. 189) bone-like tissue occurs, but it 

 appears to be merely calcified connective tissue. Occasionally the 



