Valve triangular, with concave sides and broad angles equally three-lobed, separated from the central part by septa. Central space small or absent. Puncta delicate, radiating or scattered. L. of side 62 µ. Pleistocene clay at Buckshutem, N. J. Fossil at Wildwood, N. J. T. americana, forma trigona Pant.? (Le Diatomiste, Vol. 2, p. 207.) Pl. 6, Fig. 11. (_d_) EUODIEÆ EUODIA BAIL. (1860) (derivation uncertain; apparently from euodia, fragrant, probably a euphemism) Frustule in zone view cuneate. Valve semi-lunate, coscinodiscoid. EUODIA GIBBA BAIL. Valve with rounded markings, larger and scattered at the centre, radiating at the circumference and in indefinite straight rows at the semi-radius. Delaware Bay (Mann). Pl. 5, Fig. 1. I have not seen this in the Philadelphia material. The figure is drawn from a specimen from the Gulf Stream, S. Atlantic. {35}PENNATÆ Valve zygomorphous. Structure pinnate, not concentric. Valve divided either by a true raphe or cleft or by a linear space or line imitating a raphe. Divided into three Groups: 1. _Fragilarioideæ._--Valves without a raphe; usually with a pseudoraphe or median line. 2. _Naviculoideæ._--Either one or both valves with a true raphe. 3. _Surirelloideæ._--Valves in which the raphe is concealed near the margin on one or both sides of each valve in a more or less elevated keel or wing. FRAGILARIOIDEÆ (_a_) _Tabellarieæ._--Valve symmetrical with respect to both the longitudinal and transverse axes; septate, not cuneate. (_b_) _Meridioneæ._--Valve symmetrical with respect to the longitudinal axis, asymmetrical to the transverse axis, cuneate, finely striated. (_c_) _Fragilarieæ._--Valve of varied shape, not cuneate; costate or with transverse rows of puncta. (_a_) TABELLARIEÆ Frustule in zone view rectangular, in valve view linear or linear-elliptical, sometimes constricted in the middle, symmetrical to both axes, not cuneate; with two or more septa or annuli. Chromatophores numerous, granular. _Rhabdonema._--Frustules with numerous septate partitions having one or several foramina. Transverse costæ or rows of coarse puncta. _Tabellaria._--Frustules with two to six nearly straight septa. Transverse striæ subtly punctate. _Grammatophora._--Frustules with two sinuate perforate curved septa. Transverse striæ subtly punctate. _Striatella._--Frustules with alternate partitions, septate or partly so. _Attheya._--Frustules not septate but with numerous annuli. RHABDONEMA KUETZ. (1844) (rhabdos, a rod, and nema, a thread) Frustules quadrangular, concatenate, composed of numerous septate partitions with transverse costæ or rows of puncta. Valves elliptical, with a pseudoraphe and transverse apparent costæ and punctate lines; the partitions with one or several foramina. Chromatophores in rosettes of various kinds (Karsten); usually parallel to the septa. RHABDONEMA ARCUATUM (LYNG.) KUETZ.