No. 3-] ACUTE VISION IN VERTEBRATES. 457 



ment that an area is discerned. Even with the aid of the 

 microscope the presence of an area is often very doubtful. 



The fovea is also found to have a variety of forms and posi- 

 tions. In depth it may vary from the very questionable de- 

 pression found in the guinea hen (PI. XXVIII, Fig. 36) to the 

 very deep pit of the crow or hawk (PI. XXIX, Figs. 48, 50, 52). 

 It has also been found to vary in the same species, but this 

 may be due to slight swelling produced in hardening (pigeon, 

 PI. XXVIII, Figs. 37, 38). In size it varies from the very broad 

 fovea in the human (PL XXVIII, Figs. 24, 25) to the exceedingly ' 

 sharp depression in the sparrow hawk (fovea nasalis) and the 

 horned toad (PL XXX, Fig. 56). Chievitz (31, <?) claims that 

 certain animals have also a trough-like fovea, such as the tern, 

 etc. I have not been able to procure any of the species which 

 he describes as having a trough-like fovea, but have succeeded 

 in obtaining a species from the same genus, Sterna. In a 

 macroscopical examination one would at once conclude that 

 there was a trough-like fovea present, but when sections were 

 made across this trough-like appearance, no depression was 

 found (PL XXIX, Figs. 40, 42). In my researches I have seen 

 nothing excepting the macroscopical appearance which might 

 be taken to indicate the presence of a trough-like fovea in any 

 of the animals examined. Chievitz has pictured only the 

 macroscopical view of his trough-like fovea, and in no place 

 have I found that he has made cross sections and microscopical 

 examination. In the tabulation which follows I have used his 

 descriptions, as I have not been able to examine the species 

 which he has described. 



The relations of the fovea correspond to the positions above 

 described for the area. That is, we find a fovea nasalis, as in 

 the crow, blue jay, robin, snow-bird, etc., and Sl fovea temporalis, 

 as in man, gorilla, owl, tern, hawks, etc. One or two foveae 

 may be present, but in each case where two are present the 

 nasal fovea is always deeper than the temporal. 



A very noticeable and important difference in the position of 

 the fovea in various birds has been observed. Very little vari- 

 ation is found in the position of the fovea nasalis, but the loca- 

 tion of the fovea temporalis depends wholly upon the position 



