In discussing the Naviculoid group, the general divisions of Cleve are here followed, and all diatoms having a true raphe are included. I have added the genus Epithemia and also Rhopalodia, partly because they contain a raphe of a certain kind and partly because they resemble the markings of certain of the genus Hantzschia in the following group, although in other respects there is probably no similarity.
The difficulty of combining the numerous genera into groups which are naturally affiliated is avoided in the following arrangement based on superficial similarities, and is intended merely as an artificial key. To unite all forms having a raphe and which are symmetrical with valves similar and not sigmoid, under the one genus Navicula, as has been the custom previous to the publication of Cleve's monograph, would result in associating species differing in so many respects in relation to structure of the valve and cell contents that it seems advisable to retain the new genera, especially as the original genus is likely to be still further reduced when more is known of the structure and life history of the group.
Frustules stipitate, free or parasitic. Valves cuneate, elliptical or suborbicular, dissimilar, bent along the transverse or the longitudinal axes, the lower valve with a true raphe and central and terminal nodules, the upper valve with a pseudoraphe or median line.
Rhoicosphenia.—Stipitate; valves with transverse puncta, bent along the transverse axis, cuneate, with diaphragms at the ends.
Anorthoneis.—Free; puncta radiate; valves bent slightly along the transverse axis, suborbicular.
Cocconeis.—Parasitic; valves elliptical, usually bent along the longitudinal axis; striæ punctate, transverse and longitudinal.
Achnanthes.—Stipitate; valves lanceolate or elliptical, bent along the transverse axis; striæ transverse, punctate; costæ sometimes present.
Frustule in zone view curved; valves cuneate, dissimilar, the upper with a pseudoraphe, the lower with a raphe.
Chromatophore a single plate along both valves, and one of the inner walls of the zone. Conjugation as in Gomphonema, with which it is generally associated in classification.
Valve clavate, with rounded apex and base; lower valve with raphe, a narrow axial area and slightly radiate, punctate striæ; the upper valve with a narrow pseudoraphe and parallel striæ; a short diaphragm at the ends of each valve. Length usually from 15 to 25 µ, but frequently of twice the size.
Common in Crum Creek.
Pl. 19, Figs. 25, 26, 27.
Valves dissimilar, the upper valve with an excentric axial area, the lower with an excentric raphe.
Valves orbicular, with radiating, punctate striæ, closer at the circumference, producing the appearance of a border. Axial area not reaching the ends. Frustules occur free on the sands of the sea-shore. L. 25 to 50 µ.
Belmar, N. J.
Pl. 16, Figs. 30 and 31.
Valves elliptical, dissimilar, the upper valve with a pseudoraphe and the lower with a genuine raphe and nodules, usually with a rim or annulus. Frustules epiphytic.
Cocconeis is generally considered as a degenerated form of Mastogloia, as indicated by the "obsoletely loculiferous rim." The frustules are usually bent along the longitudinal axis, probably because of the attachment to the curved stems of water-plants.
The cell contents of only a few species are known. In C. pediculus, a single chromatophore occurs on the inside of the upper valve. In conjugation, two cells open and secrete a gelatinous mass from which an auxospore is formed.
Cleve separates the forms having a loculiferous rim (Cocconeis) from those without a rim (Eucocconeis). As the rim is easily detachable, the distinction is often made with difficulty.
Valves elliptical, the upper with a linear or lanceolate pseudoraphe and coarse puncta in transverse and radiating lines; the lower valve with much finer puncta in radiating lines, a lanceolate axial area and, sometimes, a loculiferous rim.
Along the coast. Common, but extremely variable.
Pl. 16, Fig. 21 (upper valve). Fig. 18, var. ?
Upper valve with linear axial area, and transverse and radiating punctate lines which end at the border in a double row of finer puncta; lower valve with much finer puncta, a lanceolate axial area and a loculiferous rim.
Atlantic City. Common.
Pl. 16, Figs. 27 and 28.