W. M. BALE ON SOME OF THE DISCOID DIATOMS. 25- 



cause them to be easily overlooked against the coarsely marked 

 background of the valve-areolation, but in C. concinnus and 

 G. centralis they are more conspicuous, owing largely to the more 

 delicate and transparent condition of the valve. 



The key to the position of these apiculi is, however, to be found 

 in certain modifications of the valve-border which occur in the 

 vicinity, and which indeed are often obvious when it is difficult 

 or impossible to detect the apiculi themselves. These modifications 

 may take the form of a thinning away of the valve-surface (C. 

 marginatus), or an apparent notching of the margin (C. borealis, 

 C. diorama, etc.), or a sinuation of the inner edge of the thickened 

 border (G. aster omphalus) . In the last species this marginal 

 structure is very conspicuous, at least in the robust valves, and it 

 is shown in Schmidt's figures of C. biangulatus and one or two 

 others. 



In C. perforatus and C. apiculatus (at least in the areolate 

 forms) two minute notches in the extreme margin of the areo- 

 lation can in most cases be seen, and by careful examination the 

 apiculi may generally be found opposite them, but they appear 

 no more than a slight thickening of the silex, w T hich would 

 certainly never be noticed except for the marginal clue. In 

 C. marginatus the coarse radial structure of the marginal zone is 

 thinned away over two comparatively large areas, sometimes very 

 noticeably, but the apiculi themselves are difficult to make out. 



The apiculi are most fully developed in C. centralis and C. 

 aster omiAalus. They are best seen by examining the inside of 

 a large valve in which the marginal part is steeply convex, so 

 that the apiculi, which project into the valve a little above the 

 rim, can be observed without the interference of an immediate 

 background. The apiculus takes the form of a minute disc, 

 attached by a central point, and bearing a sub-globular or irregular 

 mass. The border in C. asteromphalus is usually widened in- 

 wardly so as to form an annular projection into the cavity of 

 the frustule. The extent to which this widening takes place 

 varies greatly, even in the same variety ; but whatever its width, 

 so long as it projects inwards at all, it is sinuated under the 

 apiculi, which are always uncovered, so that the sinuations are 

 deeper as the valve-border is wider. The structure would seem 

 to imply the presence in the living organism of some direct 

 communicating filaments between the apiculi of the two valves, 



