W. M. BALE ON SOME OF THE DISCOID DIATOMS. 37 



the most widely distributed species, is found in most localities with 

 six areas only, yet in some Calif ornian deposits it occurs freely 

 with up to eighteen areas, possibly more. 



A. undulatus is a species which well illustrates the tendency of 



the genus to vary in several directions, but the variations are 



so numerous and so closely linked, and their relationships so 



obvious, that they have not been made the basis of so many 



pseudo-species as might have been expected. I have noted about 



twenty-five forms sufficiently distinct to admit of their being 



separated for convenience of cataloguing, but few of them are so 



characteristic as to constitute definite varieties. In forms of 



average size, which may be considered fairly typical, the secondary 



markings are commonly about four in O'Ol mm., while in the 



var. microsticta of Grunow, there may be about seven, and in 



large forms like forma maxima Schmidt, there are only one and 



a half to two. The reticulation may be either hexagonal or 



irregular, robust or faint, and sometimes entirely wanting. The 



sub-marginal processes are said to be sometimes absent ; in fact, 



both W. Smith and Yan Heurck appear to regard this condition as 



typical, but I have not seen specimens without some trace of them. 



(The obsolete genus Omphalopelta comprised the valves with 



processes.) The processes may be very small, appearing merely 



as a slight thickening of the border, or may be placed a little 



farther in, presenting a somewhat irregular keyhole-shaped 



aspect. In many forms the secondary areas have on their 



margin a small hyaline patch in the corresponding position to 



that occupied by the processes in the primaries. On both sets 



of areas the outermost portion, immediately adjoining the margin 



proper, usually bears radial lines, being continuations of the 



boundaries of the last row of secondary markings, which, like the 



secondary markings generally, are most robust on the primary 



areas. The rim may be smooth, or may have few or many 



minute apiculi scattered over it. The puncta which compose the 



striae of the primary areas are arranged in quincunx, so that the 



striation is the same as in Pleurosigma angulatum, but those of 



the secondary areas form two sets of diagonal striae cutting each 



other at right angles, as in P. formosum. Schmidt describes as 



A. biformis valves in which these two sets of striae meet at 



rather less than a right angle, so that a third set is visible, 



closer than the other two, and crossing the area transversely. 



