32 
internally with valves is thus seen to be a valuable character, 
separating the Ganoids sharply from the Teleostei, CyclostO' 
mata (Lampreys) and Leptocardii (Amphioxus), though not 
distinguishing them from Lepidosiren, Chimcera, the Sharks or 
Bays. 
A second differential character of Miiller's restricted order 
Ganoidei is to be found in the arrangement of the fibres com- 
posing the optic nerves. In the Lampreys and the Hags 
the optic nerve proceeds direct to its orbit from the thalamus 
opticus of the same side. In a Teleostean fish the nerves 
decussate, all the fibres from one side of the brain crossing 
over to the retina of the other side. Higher Vertebrata than 
fishes possess a still more complex arrangement, a redistri- 
bution at the point of decussation of the fibres in their pas- 
sage forward to the orbit. In the human optic nerve, for 
example, there is thus formed a true chiasma (%) : some of 
the fibres are continued across direct to the retina of the 
same side ; others pass to the retina of the opposite side ; a 
few connect together the optic tracts, i. e.^ that part of the 
nerve intermediate between the brain and the chiasma ; 
lastly, some connect together the retinae or the optic nerves 
beyond the chiasma. The physiological bearing of these 
differences of structure is not unimportant, but we are here 
concerned only with their systematic value. The Ganoid 
Fishes have not the simple decussation of the Teleostei, but 
they agree with Lepidosiren, the Sharks, Rays and Chimcera, 
in having a commissure or chiasma. The arrangement of 
the fibres is less complicated than in the higher vertebrates ; 
most of the fibres pass to the retina of the same side. 
Miiller includes free gills and the presence of a gill-cover 
as a third distinctive feature of Ganoids. In the Chimcera 
Sharks and Eays (hence termed Elasmohranchii by Bona- 
parte) the gills adhere to the outer skin or wall of the gill- 
cavity, and are thus comparatively fixed, being attached at 
