KEY TO THE SPECIES
| Valves costate | biddulphiana |
| Valves not costate: | |
| Markings cellular, angles elevated into horns | favus |
| Markings cellular, angles not elevated | antediluviana |
| Markings punctate, angles with subconical processes and long spines | granulata |
| spines short | rhombus |
| spines minute | smithii |
| processes truncate, valve elliptical | turgida |
| processes truncate, valve orbicular | lævis |
| processes absent, valve divided by irregular lines | alternans |
| not so divided | reticulum |
BIDDULPHIA BIDDULPHIANA (SMITH)
Frustule quadrangular with convex ends and rounded angles. Valve elliptical with undulated sides, divided by septa into three or more sections. Processes large, rounded, globular or subconical. Zone varying in width. Surface with rounded reticulations in longitudinal and transverse rows, except at the centre where they are concentric and smaller.
Conferva biddulphiana Smith (English Botany, 1807, Pl. 1762, upper figures).
Diatoma biddulphianum Ag.
Biddulphia pulchella Gray.
Blue clay. Hoboken Tunnel. Along the coast.
Pl. [7], Figs. 1, 2, 3, and 4.
Quite variable in size and number of septate divisions. Fig. 3 is an unusual form with narrow zone, having but one row of large reticulations, evidently a young frustule.