692 Transactions of the Society. 



above tlie chambers on photograph 4. In the centre of the bubble Newton's 

 rings. The Stauroneis valve lying at the side is not free from tlie collodion. 



6. Pinnularia major. A number of exact transverse sections. Produced like 

 No. 1, X 647. The section made after method I. is of exquisite thinness 

 and flatness. It was photographed a few days after being made, therefore the 

 little air-bubbles which ought to be formed iu the chambers during the hardening 

 of the gum are visible, whilst with other sections kept for some time they have 

 been mostly absorbed by the balsam. Altogether transverse sections of twenty 

 valves can be seen, and they are lettered A to U. Of A and B only portions are 

 seen, but the others are entire. 



The focus on the left not very good ; on the right chamber opening very fine ; 

 at the lateral edge of the same an oval air-bubble in the chamber. 



D, focus in the left not suitable ; on the right median end of the chamber, 

 and median edge of its opening fairly good ; iu the extreme lateral end of the 

 chamber an air-bubble. 



E, similar section to M, but broken in the middle line ,- on the left median 

 end of the chamber pretty good. 



F, broken up and useless. 



G, a tolerably good transverse section, a little broken up ; chambers indistinct; 

 mid-rib good ; has the angle-depvession of flg. 3. 



H, only good above ; shows the fine pointing of the valve margin very sharp ; 

 in it several air-bubbles ; lower down, bjoken up. 



I, quite similar. 



K, very beautiful section ; chamber opening on the left is good, but not 

 so distinct on the right ; mid-rib is observable as a delicate cleft not quite 

 through. 



L, similar section, mid-rib burst ; on the left, chamber and its opening very 

 fine ; a few air-bubbles in the lateral end do not interfere mucli. 



M, one of the most magnificent sections, quite close to the central nodule 

 (corresponding to the line 7 in photograph 1). Tiie mid-iib is here an unusually 

 deep vertical narrow cleft ; one sees the closing envelope very distinctly (with a 

 lens) ; chamber of the upper half good, that of the lower inaistinct, because only 

 the partition-walls remain (like fig. 2). 



N, only the lower good, crushed at the top. 



O, central nodule section, hardly of use. 



P, useless. 



Q, apparently central nodule section, but injured ; focus unsuitable. 



B, focus unsuitable ; the chambers can be recognized in outline. 



S, particularly beautiful, showing the mid-rib as a right-angled bent cleft, like 

 fig. 2 ; the chambers are tolerably good in the upper half. 



T, chamber of the upper half very good, also its narrowed lateral end ; the 

 mid-rib is broken, and the lower half useless. 



U, good section, but the focus not sharp ; both chambers visible. 



The section encloses a girdle-band section. 



7. Pinnularia major. Slightly oblique longitudinal section, produced like 

 No. 1, X 650. This is another most elegant section in the direction of line 6 in 

 photograph 1, and represents the end portion from line 4 to /. This, like 

 photograph 3, is very suitable for all finer measurements under the lens. The 

 following are chiefly remarkable ; — In the region a the vertical division-walls 

 thickened below between the chambers ; the openings of the chambers ; the very 

 thin outer membrane. At the point 6 the lateral edge of a chamber opening i's 

 touched, and there appears in great strength the inner membrane (closing the 

 chamber below). At c, is a favourable image of a chamber lumen : the thin 

 outer membrane, the more than twice as tliick inner membrane, rounding off of 

 the square transverse section of the lumen at the inner membrane. 



8. Eupodiscus argus. Middle section through a valve. Produced with 

 Schroder's 1/6 dry objective, x 298. On the left is observed the inner cavity 

 in which is an air-bubble at the end. In the middle region is a faint indication 

 of vertical chamber-walls. Spine-like points are observable on the lower outer 

 surface of the valve, similar to Triceratium spines. 



9 and 10. Pleurosigma balticum. Transverse sections, produced like 1, X 669. 

 The section is made after method I. (in pure gum in collodion), and was photo- 



