W. M. BALE ON SOME OF THE DISCOID DIATOMS. 43 



circular, while A. marylandicus is three-sided, but in Atlantic 

 City slides valves of the latter species are found in which the 

 divergence from the perfectly circular form is scarcely perceptible, 

 so the objection falls to the ground. 



It is sometimes stated as a character of the genus that the 

 depressions of one valve correspond to the elevations of the other, 

 so that the frustule is radially undulated as a whole. That this 

 is not alwavs the case is evident from the fact that the two valves 

 have often a different number of areas. But I find on comparing 

 a number of species that there is considerable variation in regard 

 to the undulations. First we have forms in which the undula- 

 tions extend to and include the rim itself, so that one valve 

 necessarily fits into the other. A striking example is A. trilingu- 

 latics, in which the whole valve is so strongly undulated that only 

 three points of the margin can be seen at any one focus. Then 

 we have such species as A. undulatus and A. Heliojwlta, in which 

 the undulations do not extend outward to the margin. Apart 

 from the border itself, the sub-marginal zone is about on a level 

 throughout, but the one set of areas is inflated as much above 

 that level as the other is below it. The border itself slopes down 

 rather steeply, but the depressed areas often reach as low a level 

 as the extreme margin. Still, the width of the hoop ensures that 

 such valves may be placed with the depressions opposite each 

 other without coming into contact. Lastly, in A. splendens the 

 depressions do not reach as low as the margin, while the eleva- 

 tions rise considerably above it ; even with a narrow hoop, 

 therefore, there is no question of the depressed areas of opposite 

 valves clashing. 



According to the definitions of Ralfs and Yan Heurck, a 

 character of the genus is the division of the valve into equal 

 cuneate segments, which would exclude from it the A. hispidus 

 Grunow (Van Heurck, Synopsis, PI. 123, fig. 2), a species which 

 is described as having narrow elevated compartments alternating 

 with wide depressed ones. I believe, however, that the so-called 

 elevated compartments of A. hisjndus are not compartments at 

 all in the same sense as those of Actinoptychiis ; neither are they 

 elevations, but only appear so owing to having depressions on 

 each side of them. The valve is a shallow cone, by far the 

 greater part of which is occupied by about eight or nine broad 

 radial cuneate areas, all of which are depressions. The linear 

 rays or ridges are simply parts of the surface not included in the 

 depressions, but dividing them. These rays slope down evenly 



