MICROSCOPICAL APPEARANCE OF THE VALVES OF DIATOMS... 219 
On the exterior side of this line is to be seen a large flap, exces- 
sively convex, and on the interior side a smaller stunted flap. 
Fig. 45 shows this arrangement in section and in projection. 
The small flap is always of relatively small breadth, and its 
curvature very trifling; it is curved, doubly curved, or straight 
on its external surface, according to the contour of the raphe with 
which it is in contact, while its opposite side is always straight, or 
has merely the slightest curve. Fig. 46 shows some of the com- 
mon forms of the smaller flap of the Amphore, Its surface is 
Fig, 46. 
a, Central nodule (7odule central) 
6, Nodules at ends of raphe (odules terminaux du raphé) 
smooth or bears, in general, different designs to those which orna- 
ment the large flap. 
In a complete but very young frustule the two small flaps touch 
each other on their free edges, while on the opposite side the two 
large flaps are equally united by a line of unbroken contact. This 
is the reason that when an Amphora is placed in the focus of a 
microscope under a high power, that we see simultaneously the 
two raphe as well as its two smaller flaps—that is to say, if it is 
lying upon its back, or on the line of junction of the large flaps. 
If the frustule be turned over it only shows the two large flaps, or 
rather the posterior parts of these, as well as their line of contact, 
which, however, is often very difficult to distinguish. 
No trace of raphe, small flaps, or nodules are then visible, unless 
it be through the whole thickness of the frustule, by virtue of its 
transparency, under an objective having considerable penetration, 
a circumstance not infrequently occurring with these small diapha- 
nous objects. 
In the case of adult Amphorze—as with all other diatoms—each 
valve is united to a hoop which encircles it round the whole peri- 
phery in such a manner that the careful observer will not fail to 
notice it, if he examine the frustules some time —- the com- 
mencement of subdivision. a 
