574 William Patten 



pigment below. Just before reaching the tapetiim, the image of any 

 object in front of the pupil will be seen with the greatest distinct- 

 ness, diminishing in definition according as the objective of the micro- 

 scope is raised or lowered. But the rods also He just above the tape- 

 tum, so that upon them the image must be formed. A remarkable 

 phenomenon may be observed, by focusing between the argentea and 

 the place where the image formed by the lens is seen with the greatest 

 distinctness , for there one sees a double image, less dis- 

 tinct towards the argentea, but iucreasing in sharpness 

 towards the focal point of the ey e, where the two images 

 ultimately fuse to form a single one. The only explanation 

 I bave to offer for the origin of this second image, is that it is a re- 

 flected one of the first, formed by the curved surface of 

 the argentea. A plain mirror would never reflect an image formed 

 by a lens, since the rays of light would be dispersed ; neither would the 

 image be reproduced by a concave mirror, unless the curvature was 

 such that the divergent rays coming from the lens impinged upon the 

 reflecting surface, at right angies to the tangent at that point. In that 

 case each reflected ray would coincide with the incident one, and a 

 reflected repetition of the lenticular image would be reproduced, both 

 being formed at the same point. The exact relation between the focal 

 distance of a lens like that of Pecten^ and the radius of the concave 

 mirror which would again unite the rays , has not been determined 

 (PI. 32, fig. 149). 



Anatomy of the Eyes. 



The mantle of Pecten would serve very well as a type , since it 

 possesses ali the structures fouud in any Lamellibranchiata , with the 

 exception of the varìous forms of faceted eyes so characteristic of Arca. 

 The shell fold is separated from the Ophthalmie, by a very deep furrow, 

 the bottom of which is occupied by a continuous, double ridge of thick- 

 ened cells which scerete the cuticular covering of the shell. The outer 

 of these ridges, both of which extend the whole length of the mantle, is 

 large and heart-shaped, while the other is high and narrow {PI. 28, e. g.). 



The Ophthalmie fold projects considerably above the edge of the 

 mantle, and is deeply forked at its free end. From the inner wall of its 

 base, arise the innumerable tentacles, and, at certain intervals, the 

 stalked eyes, which it is our purpose to consider. The tentacles are usu- 

 ally situated between the eyes and the Ophthalmie fold (PI. 28, fig. 7j, 

 but occasionally the re verse may be the case. 



