Valve with marginal striæ well marked, each third or fourth costa more robust than the others. Central part finely striated, the striæ punctate, radiating. Fresh water. Pl. 2, Fig. 7. The form here figured is probably the variety _radiosa_ Grun. and is from a New England specimen. It is quite likely to occur in this locality. CYCLOTELLA OPERCULATA (AG.) KUETZ. Frustules in zone view undulated. Angles rounded. Marginal costæ alternating with minute spines; centre nearly smooth, depressed, convex or flexuose. Fresh water. Pl. 2, Figs. 5 and 6. The figure is drawn from a specimen from Boston, Mass., H. L. Smith Type Slide No. 107, marked equivalent to _C. minutula_ Wm. Sm. CYCLOTELLA ANTIQUA WM. SM. Marginal costæ alternating with thick puncta; centre finely granulate with subtriangular elevations. Frustules in zone view rectangular. Blue clay. Pl. 2, Fig. 11. The form corresponds to the original specimens of Wm. Smith in the deposit of Stavenger, Norway. The genus Cyclotella comprises about seventy specific names, many of which may be referred to other genera, while some of Ehrenberg's are incapable of verification on account of the small size of the figures and the lack of sufficient description. About half of the forms are marine. The fresh-water species are usually found living in more or less stagnant water or in pools contaminated with drainage, being an exception to the general rule that diatoms are more abundant in water free from deleterious matter. {21}COSCINODISCUS EHR. (1838) (coscinon, a sieve, and discus) Frustules solitary, cylindrical, compressed; valve circular or elliptical; surface flat or sometimes convex near the border; markings more or less angular, radiating, sometimes fasciculate; border usually well defined. Central space, if present, hyaline, sometimes surrounded with a rosette of large cells. Chromatophores round, angular or irregular discs usually without pyrenoids (Karsten). Rattray's classification is here followed, so far as it refers to our species. _Excentrici._--Valves circular; central space absent; markings angular, in oblique, decussating rows. _Lineati._--Central space absent; markings angular, oblique decussating rows straight. _Fasciculati._--Markings fasciculate, or sometimes only near the border. _Radiati._--Markings rounded or angular, more or less radiate. _Elaborati._--Valves elliptical, markings rounded. EXCENTRICI COSCINODISCUS EXCENTRICUS EHR. Valve with a hyaline excentric space from which proceed, usually in six directions, rows of polygonal markings decreasing toward the narrow, coarsely striated border, the rows appearing convex toward the centre. Apiculi at unequal distances apart. Quite variable in size. Common in the blue clay and along the coast. Pl. 2, Figs. 14 and 20. Fig. 20 is probably var. _perpusilla_ Grun. (Diat. Fr. Jos. L., Pl. 4 (D), Fig. 7). LINEATI COSCINODISCUS LINEATUS EHR.