307 



Of all the animals which I liave been able to tabulate (227 species) no area 

 was found in 26, a round area was perceived in 183, and a band-like area in 55. 

 Of this number, 120 possessed a simple fovea, 12 a double fovea, and according 

 to Chievitz 25 had a through-like fovea. 



Mammals do not as a rule possess a fovea, but generally have an area. Of 

 the 51 species tabulated 10 were found in which no area was demonstrated ; 33 

 had a round area and eight a band-like area. Only 18 species (all primates) pos- 

 sessed a well-defined fovea. 



In the 104 birds all had a clear fovea excepting one, the common chicken. 

 Why the chicken does not have a fovea when it is present in all the nearest allied 

 forms remains a query. A round area was found in every case, and in 36 a band- 

 like area was also observed. Ninety-one had a simple fovea, twelve double 

 fovea, and twenty-two the questionable trough-like fovea. 



Among reptiles a well-defined fovea has generally been found in the lizards 

 and crocodiles, but it has not been observed in the snakes and turtles. Of the 

 twenty-eight species examined only three were found which did not possess an 

 area, while twenty-three had a round area and three band-like ones. A round 

 fovea was seen in the lizards tabulated and a trough-like fovea in the crocodiles. 



A fovea has been observed in only two of the fourteen amphibians tabulated. 

 Chievitz reports a trough-like fovea in Bufo calamila and Hulke a simple fovea 

 in Bufo vulgaris. I have not found a fovea in any of the amphibians which I 

 have examined. In the tabulation, three of the number had no area, three had 

 a round area, and eight possessed a band-like one. I have found the band-like 

 area common to frogs and toads. 



In fishes the absence of a fovea is the rule. In the thirty species given a 

 fovea was observed in but five, and no area in ten. In these ten, however, the 

 material at hand was not suflicient to warrant a definite statement. Of the 

 twenty-six fishes I have examined only one was found with a fovea. This was the 

 pipe fish {SiphoMoma fuscum). 



When one compares the retinas in the different vertebrates he finds a 

 marked diversity. A great difference is noticed in the relative thickness of the 

 different layers. But the most marked change is noticed in the rod and cone 

 layer. Comparing the diameter, length, shape and relative number of the rods and 

 cones we find that fishes, frogs and mammals possess the longest rods. In mam- 

 mals the rods have the smallest diameter, and in frogs the greatest of any of the 

 vertebrates. In birds they are comparatively short and thick. The cones are the 

 longest in some of the reptiles (chameleon) and of greatest diameter in am- 

 phibians and mammals. They have about the same length in birds and am- 



