Valve nearly symmetrical, obovate, with transverse costæ about 4 in 10 µ, alternating with double rows of coarse puncta; median line distinct, linear. L. 43 µ. _Podocystis americana_ Bail. Hell Gate, N. Y. Pl. 40, Fig. 6. CYMATOPLEURA WM. SM. (1851) (cuma, a wave, and pleura, a side) Valve elliptical; surface transversely undulate, with short, marginal costæ. Frustule in zone view linear, with undulated sides. Auxospore formation as in Surirella. CYMATOPLEURA SOLEA (BRÉB.) WM. SM. Valve oblong, with cuneate apices, constricted in the middle; costæ about 6 in 10 µ; striæ, 10 in 10 µ; pseudoraphe scarcely visible. L. 50-300 µ. Blue clay. Common in the Hudson River. Pl. 34, Figs. 8 and 9. CYMATOPLEURA ELLIPTICA (BRÉB.) WM. SM. Valve elliptical; marginal costæ short, 3 in 10 µ; striæ delicate, 18 in 10 µ; undulations four or more. L. 70-140 µ. Blue clay. Pl. 37, Fig. 1. _Forma spiralis._--Valve ovate, swelled into curved ridges at the lower end, with a contraction of the valve. Port Penn, Delaware River. Pl. 37, Fig. 2. CYMATOPLEURA MARINA LEWIS Frustule linear, with numerous undulations, ends apiculate; valve linear-lanceolate, with acute ends; striæ transverse, punctate at unequal intervals, from 16-18 in 10 µ. L. 43 µ. East River, N. Y. Pl. 37, Figs. 3 and 4. Lewis states that the ends are more or less truncate. I do not find them so. CAMPYLODISCUS EHR. (1841) (campulos, curved like a saddle) Valve orbicular or sub-orbicular, with costæ or punctate rays converging from the circumference toward the hyaline centre, which sometimes appears like a pseudoraphe. Frustule of two saddle-shaped valves at right angles to each other. The zone view may be of almost any shape according to position. Endochrome consists of two bands, each lining the inner surface of each valve. Auxospore and conjugation unknown. {130}CAMPYLODISCUS ECHENEIS EHR. Valve sub-orbicular, saddle-shaped; costæ indistinct, short, marginal; rows of round or elongated puncta converge toward the lanceolate, hyaline median space. Diam. 80-140 µ. _Campylodiscus argus_ Bail. Blue clay. Reservoir at Thompson and Twenty-sixth Sts., Phila. Pl. 37, Fig. 6. This form, usually considered as brackish and marine, is occasionally found in fresh water. According to Deby, it is fossil in the "Champlain deposit of N. A." CAMPYLODISCUS HIBERNICUS EHR. Valve irregularly orbicular; costæ, 40-60, about 2 in 10 µ, wide at the margin and attenuated toward the centre which is somewhat quadrate; the radials rough with minute apiculi. Pensauken, N. J., artesian well. Pl. 37, Fig. 5. {131}APPENDIX COLLECTION AND PREPARATION OF DIATOMS It is assumed that every student of the Diatomaceæ has a general knowledge of the collection, preparation, mounting and examination of material. For the novice, however, the following methods, used by the author for many years, may be of service.