FISHES 



309 



this species the triple cone consists of a large central cone with two smaller ones 

 on either side of it ; the quadruple cones are formed by three small cones 

 grouped symmetrically around a large central cone. A physiological explanation 

 of the significance of double, twin or multii)le cones has not yet been advanced.^ 



In most Teleosteans the retina shows a circumscribed region 

 where it is thicker and more liighly j^acked with visual elements than 

 is the remainder of the fundus, constituting an ill-defined area centralis ; 



Figs. .3.")1 to 3.')8. — The Fovea of Teleosts. 



Fig. 3")1. — Section through the fovea of the blenny, Blennius. 



Fig 



3o2.— The fovea, Fo, of the sea- 

 horse, Hippocampus. 



Fig. 3.53. — The fovea, Fo, of the sea- 

 bass, Serranus (Kahmann, v. Graefes 

 Arch. Ophthal). 



here the density of the cones, the bipolar cells and the ganglion cells 

 is increased. In the guppy, Lebistes, and the killifish, Fundulus, the 

 area is apparently duplicated, one lying axially, another ventrally 

 (Vilter, 1948). In a number of species, particularly the agile and lively 

 inhabitants of the littoral zone, a fovea is present in addition in the hori- 

 zontal meridian of the temporal retina (Kahmann, 1934-36) (Figs. 351-3). 

 Among Fishes this is unique to Teleosteans. It usually takes the form 

 of a shallow pit, inferior in its retinal differentiation to the correspond- 

 ing area in lizards, Birds and Primates, but it may be well formed (pipe- 

 fish, Syngnathus — Krause, 1886 ; the labrid, Jul is, and the blennj-, 



1 p. 253. 



Syngnathun 



