[Illustration: PLATE 30] PLATE 31 NAVICULA 1 Navicula elegans Wm. Sm. 101 2 Navicula elegans var. cuspidata Cl. 101 3-4 Navicula grevillei (Ag.) Cl. 99 5 Navicula libellus Greg. 99 6-7 Navicula palpebralis Bréb. 101 8 Navicula rhyncocephala Kuetz. 97 9 Navicula cryptocephala Kuetz. 97 10 Navicula longa (Greg.) Ralfs. 97 PINNULARIA 11 Pinnularia brébissonii (Kuetz.) Cl. 107 12 Pinnularia borealis Ehr. 109 13 Pinnularia divergens var. elliptica Grun. 107 EPITHEMIA 14 Epithemia turgida (Ehr.) Kuetz. 111 15-21 Epithemia argus Kuetz. 111 16 Epithemia argus var.? 111 17 Epithemia muelleri A. S. 111 18 Epithemia zebra var. proboscidea (Kuetz.) Grun. 112 19 Epithemia gibberula var. producta Grun. 112 20 Epithemia musculus Kuetz. 112 22 Epithemia musculus var. constricta (Bréb.) V. H. 112 RHOPALODIA 23 Rhopalodia gibba (Kuetz.) Mueller 112 24 Rhopalodia ventricosa (Kuetz.) Mueller 113 [Illustration: PLATE 31] PLATE 32 NITZSCHIA 1 Nitzschia circumsuta (Bail.) Grun. 118 2 Nitzschia plana Wm. Sm. 117 3 Nitzschia granulata Grun. 116 4 Nitzschia navicularis (Bréb.) Grun. 116 5 Nitzschia panduriformis var. minor Grun. 117 6 Nitzschia apiculata (Greg.) Grun. 117 7 Nitzschia tabellaria Grun. 119 8 Nitzschia tryblionella Hantzsch 116 10-11 Nitzschia bilobata Wm. Sm. 118 12 Nitzschia litoralis var. delawarensis Grun. 118 13 Nitzschia acuminata (Wm. Sm.) Grun. 117 14-25 Nitzschia amphibia Grun. 122 15 Nitzschia palea (Kuetz.) Wm. Sm. 122 16 Nitzschia fluminensis Grun. 120 17 Nitzschia obtusa var. scalpelliformis Grun. 121 18 Nitzschia linearis (Ag.) Wm. Sm. 122