VISUAL ANGLES AND FIELDS 293 



a field, and it amounts to 25° in some star-gazers (genus Uranoscopus) , 

 30-40 in some of the blennies, and to still higher values in other star- 

 gazers (Astroscopus*), in many batoids and flatfishes, and in such for- 

 ward-and-upward-lookers as the toadfish, Opsanus tau. Purely accidental 

 and of little value on the other hand, is the narrow posterior binocular 

 field which many fishes possess. The nasal retina is too crude for them 

 to make any real use of such a field. A ventral or downward binocular 

 field is useful to pelagic fishes, and some surface forms (needlefishes, 

 halfbeaks, flyingfishes, the look-down [Vomer setipinnis] etc.) have their 

 eyes canted downward to produce one; but in most fishes the angle 

 between the optic axes in the vertical plane is concave upward. 



Amphibians nearly all have a binocular field, wider in anurans than in 

 urodeles and much reduced or absent in some of the latter; but no exact 

 determinations appear to be on record. The horizontally oval pupil of 

 most frogs and toads would tend to extend the binocular field a bit, but 

 its primary meaning is probably in connection with periscopy. 



The reptiles show less variation than the fishes (see Table IX, 

 next page). The crocodilians have about 25° of binocular field. In the 

 turtles, one extreme is given by the herbivorous Testudo (18°), and the 

 other by the snapping turtle, Chelydra (38 ). Two-thirds of the snap- 

 per's food is animal, and half of this consists of game fishes. The snap- 

 ping turtle strikes its prey like a snake, and thus has special need of the 

 good distance-judgment which binocularity confers. The lizards have the 

 strongest laterality of the eyes, with binocular fields of only 10 to 20 

 as a rule. Though most species are predaceous their prey is small; but the 

 lizards themselves have much to fear from predaceous birds and mam- 

 mals, and have therefore retained their periscopy. The monitors (Varan- 

 idae) are big enough to fear nothing, however, and anticipate their sup- 

 posed descendants, the snakes, with values of 30 or more. In snakes the 

 binocular angle ranges mostly between 30 and 40 , with higher values 

 in strongly eye-minded, striking snakes such as Dryophis, whose key-hole 

 pupil is a clever device for widening the binocular field without this being 

 (as it is in Zamenis flagelliformis) at the expense of periscopy. The river- 

 snakes, Acrocbordus javanicus and Cerberus rbyncbops, have an exten- 

 sive binocular field which is directed largely upward, but these forms 

 seem not to be guided by vision at all. Kahmann states that they 'tongue' 



*The species of Astroscopus stare fixedly upward. In this genus the eye-muscles are much 

 reduced, and portions of one or more of them have been converted into a huge electric 

 organ, occupying the enlarged orbit in which the small eye has been crowded forward. 



