52. Striae composed of puncta in a double series, interrupted near the 



margin of the valves so that a longitudinal marginal line is formed; 



attached Gomphoneis 



PI. 7, Fig. 3 



52. Striae composed of puncta in a single series; attached on branched 

 stalks or floating Gomphonema 



PI. 6, Fig. 21 



53. Frustules with septa 54 



53. Frustules without septa 55 



54. Frustules quadrangular in girdle view, usually in zig-zag chains, 

 longitudinally septate, the septa with openings in the center and 



at the poles Diatomella 



PI. 7, Fig. 4 



54. Frustules narrowly rectangular in girdle view, naviculoid in valve 

 view; septum with large, central opening and parallel linear open- 

 ings which form two lateral series of minute, transverse canals; 

 septum forming a ring below valve surface Mastogloia 



PI. 7, Fig. 5 



55. Raphe extending within a siliceous rib-like thickening 56 



55. Raphe not bordered by siliceous ribs 58 



56. Valve with an enlarged, undecorated central area on the region of 



the central nodule; frustule broadly elliptic; valve costate Diploneis 



PI. 5, Fig. 13 



56. Valve without an enlarged, clear area in the central region; valve 



not costate; frustules linear-lanceolate 57 



57. Central nodule greatly elongated, the raphe appearing in two rela- 

 tively short sections in the apical region Amphipleura 



PL 6, Fig. 23 



57. Central nodule shorter, with two siliceous ribs extending toward 



the apices; raphe lying within the ribs Frustulia 



PI. 7, Fig. 6 



58. Valves with wings, bearing a sigmoid keel, '8'-shaped in girdle 



view Amphiprora 



PI. 6, Fig. 12 



58. Valves without a keel 59 



59. Transverse valve markings interrupted, thus with longitudinal lines 

 paralleling the margin "0 



59. Transverse markings not so interrupted 62 



60. Interruption of transverse lines forming a zig-zag pattern or 

 line Anomoeneis 



PI. 7, Fig. 8 



60. Interruptions not forming zig-zag pattern or line 61 



61. Transverse markings formed of puncta in a discontinuous 



pattern Neidium 



PL 7, Fig. 11 



61. Transverse markings of continuous lines Caloneis 



PL 7, Fig. 21 



62. Clear area in longitudinal axis sigmoid Scoliopleura 



PL 7, Fig. 20 



62. Clear area of axial field not sigmoid 63 



[948] 



