FISHES 



321 



THE HOLOSTEAN EYE 



TWO EXTANT REPRESENTATIVES are all that leniaiii of the very 

 ancient group of Holosteans, both confined to North American waters 

 — the bow-fin, Amia, and the gar-pike, Lepidosieus. As the progenitors 

 of Teleosteans, it is to be expected that their eyes resemble the 

 teleostean type (Ziegenhagen, 1895 ; Franz. 1934). 



A m ia 



Figs. 374 and 375. — The Visual Elements of Am/a 



Lepidosieus 



Fig. 374. 



Fig. 375. 



Fig. 374. — Tlic cones of Amia ; a single cone and a double cone. 

 Fig. 375.— a rod of Amia ( X 1,000) (Gordon Walls). 



The sclera has a complete cup of hyaline cartilage ; the cornea is 

 tinted a yellow colour (Walls and Judd, 1933) and the laminated 

 substantia propria is homogeneous ; the annular ligament at the angle 

 of the anterior chamber is marked. The choroid has typical teleostean 

 features with an argentea, a large choroidal gland (in Amia only), a 

 falciform process and a campanula with an ectodermal muscle ; there 

 is a dorsal suspensory ligament and (as in some Teleosts) a membrana 

 vasculosa retinae, the vessels of which, however, enter at the mid- 

 ventral point of the ora. The iris, over which the argentea is prolonged. 



S.O. — VOL. I. 



