Edwards.] 256 [May 17, 



vision. The retina is by no means simply a membranous expansion 

 of nerve substance, but a most complicated structure. Without 

 dwelling upon the arguments in proof, I would simply say that its 

 outer layer contains alone the percipient elements, called from their 

 shape the rods and cones. These stand crowded together after the 

 manner of a mosaic, at right angles to the black pigmented surface 

 against which they lie, or rather in which they are bedded. The outer 

 portion of these wonderfully minute little rods and cones is now found 

 to be composed of a pile of plates of a refracting material separated by 

 a less refractive intermediate substance, like a pile of glass plates sepa- 

 rated by air. In contact with these come the ultimate fibrillar of the 

 optic nerve fibres, and in some way the action of the light streaming 

 through this pile of plates stimulates the nerve to a sensation of 

 light to go to the brain. This plated or layered structure of the rods 

 and cones is universal in the vertebrate eye. The portion of the 

 compound or facetted eye which corresponds to the rods and cones is 

 the nerve substance, or its representative, behind the vitreous cone in 

 the pigmented tube, and here also this plate structure has been found 

 in the insects and crustaceans. Thus, then, not only are all eyes so 

 formed as to be adapted to the same laws of light, in having refract- 

 ing media, in the plane of whose focus a recipient organ turns light 

 into nerve sensation, but the percipient elements of this receptive or- 

 gan, the retina, are also the same, perfectly establishing the unity of 

 design i n a ll visual organs of men and lower animals. 



Notice of an Undescribed Form of Pleurosigma; Family 

 Diatomace.e. By Prof. Arthur Mead Edwards, New 

 York. 1 



With no wish to add another synonym to the already long lists 

 which burden our overcrowded botanies, but simply with the de- 

 sire of bringing to the notice of such biologists as are students of 

 the Diatomacese, what I presume to be new, or, at least, unpublished, 

 I venture to describe, and at the same time attach a provisional 

 name to, a form of plant belonging to the family mentioned, and 

 genus Pleurosigma of W. Smith, which I discovered some years since 

 in a gathering I made at that time in a salt marsh situated near, and 

 bordering, Coney Island, one of the small islets lying upon the ocean 

 side of Long Island, New York. 



1 This paper was presented April 5th, 1S71. 



