CLA SSIFICA TION 



277 



is no reason for rejecting the old view that they are the two halves 

 of the girdle, comparable to those found in Holocephali and 

 Pleuracanthodii. In the Dipnoi and Elasmobranchii only the two 

 halves become intimately fused together, as in the higher terrestrial 

 Vertebrates. 



The living Teleostomes differ from the Dipnoi (and the 

 Chrondrichthyes) in several important respects besides those 

 mentioned above : the branchial lamellae are supported by a double 

 series of rays on each arch ; the brain is characterised by the pre- 

 ponderance of the hind- and 

 mid-brain over the fore-brain and 

 the small development of the 

 anterior region of the fore-brain 

 (telencephalon), which retains a 

 simple epithelial roof (Figs. 283, 

 353). The basal ganglia thicken 

 below ; but there are no paired 

 cerebral outgrowths. There is 

 no cloaca, the urinogenital open- 

 ing being behind the anus. 



The ova are relatively small, 

 and generally extremely numer- 

 ous. Those of the lower Teleo- 

 stomes (Polypterus, Lepidosteus, 

 Acipenser, Amia) are provided 

 with comparatively little yolk, 

 and are quite or nearly holo- 

 blastic in cleavage. The larvae 

 have organs of fixation, in the 

 ehape of paired suckers in front Ventral yiew of ^ md fli| 



Of the mOUth, unlike those Of of Lepidosteus osseus, L. The skeleton is com- 



the Dipnoi and Amphibia which 

 are ventral and behind the mouth. 

 In the Teleostei the yolk is rela- 

 tively very large in amount and the cleavage meroblastic. Fre- 

 quently their eggs float freely in the sea with the help of an oil- 

 globule of light specific gravity (Dean [105], Agassiz [3], Balfour 

 [30], etc.). 



The orders Ganoidei, Ctenoidei, and Cycloidei of Agassiz 

 (p. 210), founded merely on the structure of the scales, were shown 

 by Johannes Miiller to be to a great extent artificial. Following 

 rather the example of Cuvier, and trusting chiefly to distinctions of 

 internal anatomy, Miiller determined the limits of the group 

 Ganoidei anew [307], purging it of its foreign elements. He 

 divided Agassiz's three orders into two sub-classes, the Ganoidei 



FIG. 246. 



