This form is not usually described as having punctate areolæ, but it does not apparently differ from other forms of Pyxidicula of Ehrenberg as described by Kuetzing (Species Algarum, pp. 21-23), including _P. areolata_. In fact, it differs from Stephanopyxis, which is also sometimes punctate, only in the absence of spines. In fossil deposits the absence of an easily detached stratum is not significant. The difference, except in size, between it and _P. mediterranea_ Grun. (V. H. S., Pl. 95, Figs. 15 and 16), I am unable to determine. Although many species of Meloseira are fresh-water, the habitat of the group Meloseirinæ is, in general, marine. It more nearly coincides in structure and development with other algæ not diatomaceous, the siliceous envelope constituting its most distinctive feature. As we proceed in the classification, the structure both of the frustule and contents becomes more complicated. (_b_) COSCINODISCINÆ 1. _Cyclotella._--Valve with two concentric divisions of different structure, one a wide border and the other a central surface. 2. _Coscinodiscus._--Valve areolate or punctate, with a narrow border of the same structure. CYCLOTELLA KUETZ. (1833) (cyclos, a circle) Frustules single or geminate, cylindrical, short, in zone view rectangular or with undulating sides. Valve usually with smooth or punctate striæ, centre sometimes bullose, smooth, or with granules scattered or radiating. Chromatophores numerous along the valves (Pfitzer). CYCLOTELLA STRIATA (KUETZ.) GRUN. Valve 30-80 µ in diam., with coarse striæ, 7-12 in 10 µ, centre coarsely punctate and bullose. _Coscinodiscus striatus_ Kuetz. _Cyclotella dallasiana_ Wm. Sm. Common in the blue clay. Pl. 2, Fig. 9. CYCLOTELLA MENEGHINIANA KUETZ. Frustule in zone view rectangular, undulated; valve, 10-20 µ in diam., marginal striæ robust and transversely punctate, centre radiately punctate. _Cyclotella kuetzingiana_ Wm. Sm. (not Thwaites). Crum Creek. Pl. 2, Fig. 8. {20}CYCLOTELLA MENEGHINIANA, VAR. STELLIGERA CL. AND GRUN. Differs from the type in the coarse radiating lines at the centre. Broomall Lake, Media. Pl. 2, Fig. 4. CYCLOTELLA MENEGHINIANA, VAR. STELLULIFERA CL. AND GRUN. As in type but with the central rays granulate. Broomall Lake, Media. Pl. 2, Fig. 12. CYCLOTELLA STYLORUM (BR.?) V. H. Margin striated, the alternate striæ thickened near the border, producing an appearance of subquadrate cells. Centre faintly granulate, the outer border of which is encircled by 10-12 puncta, each of which is surrounded by a small hyaline space. Blue clay. Rare. Van Heurck gives this form doubtfully as a variety of _striata_, while De Toni makes it synonymous with it. Van Heurck's figure is not that of Brightwell, but as the specimen above described is, I believe, exactly the same as Van Heurck's, I retain his name. Pl. 2, Fig. 10. CYCLOTELLA COMTA (EHR.) KUETZ.