iSpi.] NEW-YORK MICROSCOPICAL SOCIETY. 63 



in the sense of its occupying the centre of the section of the valve. 

 To understand its position it must be remembered that in section 

 the valve is V-shaped (Fig. i); No. i being the inner, No 3 

 the extreme outer side of the valve, and No. 2 immediately under 

 No. 3, and optically, but not structurally, a part of it. When I 

 say optically a part, I mean that had I not seen them separated 

 I should never have ventured to declare them different layers, 

 Nos. I and 2 are comparatively robust in structure, while No. 

 3 is exceedingly fragile and often seen lying in strips on the face 

 of the valve; and it is from the fact that under these circumstances 

 No. 2 is still found sound I have been able to differentiate the 

 two. This difference of curve between the two sides of the valve 

 is seen to run through all the species of Fleurosigma, although in 

 none of them are they so pronounced in character as in F.for- 

 mosum. This difference always corresponds with a difference of 



appearance, and although I do not believe the variation, structu- 

 rally, between the two sides of some of them is much, it must be 

 enough to account for the difference. 



The first four of the series of lantern slides sent to illustrate 

 this paper are numbered i, 2, 3, and 4. and represent the three 

 layers of the valve of P. formosum; Nos. i and 2 being taken 

 from the inner side and showing it to be nearly flat in section. 

 The structure of this layer seems to be a square grating set 

 lengthways on the valve, and its peculiarity is that on the same 

 plane the focal images are formed in the alternate squares of the 

 grating only, giving the usual appearance of the diagonal mark- 

 ings. On the outer focus the alternate holes are red, the inter- 

 spaces white, as in slide No. 12; but on focussing inward the 

 white interspaces turn into green " beads," and the outer red inter- 

 spaces become white. This red and green is not merely a negative 

 and positive image of the same structure, but formed in entirely 



