482 SLONAKER. [Vol. XIII. 



to the naked eye and is demonstrated only in vertical sections 

 by a slight and gradual increase in thickness, principally in the 

 inner nuclear layer and in the closer arrangement of the nerve 

 cells. The position of the area is outlined in PI. XXVII, 

 Fig. 18, as found in Rana catesbiana. PI. XXX, Fig. 62, 

 represents the vertical section through the area. 



Fishes. 



Fishes seem to be characterized, as a rule, by the absence of 

 both a fovea and a well-defined area. Nothing is visible to the 

 naked eye excepting in a few cases, which will receive special 

 mention. If sections of the eye, however, are subject to micro- 

 scopical measurement, an oblong or oval region, slightly thicker 

 than the rest of the retina, is found located on the temporal 

 side and a little above the center. In fact, the whole upper 

 half of the retina is somewhat thicker than the lower half in 

 all fishes which I have examined. That region indicated above, 

 however, is the thickest, and I have designated it the area 

 centralis. It also corresponds in position to that of the fovea 

 when a fovea is present. Some of the material at hand was not 

 sufficient to demonstrate clearly the presence of such an area. 

 Such cases I have indicated as doubtful. PI. XXX, Fig. 63, 

 represents a section through the area of the flounder (Para- 

 lichthys dentatus), but no increase in thickness is visible in so 

 small a portion of the retina. 



Krause (37) has described the presence of a round area and 

 shallow fovea in Syngnathus typhle, and Carriere (16) has 

 described and pic^red a similar area and fovea in Hippocampus. 

 Gulliver (15) has described a round area and shallow fovea in 

 Pagellus centrodonpus (.'). Schiefferdecker (38) has described 

 a similar area and fovea in Pleuromectes platessa. I have not 

 been able to procure any of these species, but have found an 

 area and fovea in another species. 



Pipefish (Siphostoma fuscum). 

 The eye of this fish being so small, I have not attempted a 

 macroscopical examination. The area and fovea are, however, 



