_Fasciculati._--Markings fasciculate, or sometimes only near the border.
_Radiati._--Markings rounded or angular, more or less radiate.
_Elaborati._--Valves elliptical, markings rounded.
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.
Fig. 20 is probably var. _perpusilla_ Grun. (Diat. Fr. Jos. L., Pl. 4 (D), Fig. 7).
Valve circular, markings hexagonal, cells in parallel rows. Border narrow, cellular.
Blue clay and Atlantic coast. Not common.
Valve flat, markings rounded, distant, radiate, decreasing toward the border which is coarsely striate. Quite variable in size and in the distance between the markings.
Valve usually not quite circular; markings smaller than in nitidus and fasciculate near the border.
Various intermediate forms between nitidus and nitidulus occur.
Markings polygonal, irregular at the centre, but forming numerous fasciculi radiating { 22}toward the border, the rows parallel to the central row of each fasciculus. Border narrow with fine striæ; apiculi often present between the fasciculi.
Blue clay and along the coast. Very common in the water supply of Philadelphia and Camden, where the diameter seldom exceeds 40 µ and the markings on the semi-radius are 10 in 10 µ.
Markings larger than in C. subtilis, equal, forming usually ten fasciculi, each beginning near the semi-radius and containing ten parallel rows of granules.
Common in the blue clay and sparingly along the coast.
Forms are found intermediate between C. subtilis and C. denarius, as shown in Fig. 15.
Markings angular, 10 in 10 µ, decreasing toward the border, fasciculate. Apiculi large, twelve or more, usually inserted at the middle of each fasciculus, and extending into the interior of the cell. The apiculi in outline resemble the heads of horse-shoe nails, and are seen with difficulty except when the valve is examined from the inner side. Border narrow, striated. Diam. 70 µ.
Rattray's description of _C. polyacanthus_ var. _intermedia_ Grun., from Cape Wankarema, Siberia, gives the diam. as 60 µ, and there are about 7 markings by actual count in 10 µ in Grunow's figure (Diat. Fr. Jos. Land, Pl. 3 (C), Fig. 25). The apiculi are more numerous, but there appears to be little doubt of the general similarity. The Philadelphia form is abundant in the Pensauken well deposit at a depth of 33 ft. The apiculi become quite distinct in slides stained with silver nitrate by Mr. F. J. Keeley; they are distinct from small apiculi sometimes evident between the fasciculi. The specimens in the Pensauken deposit are mingled with other forms which cannot be distinguished from _C. subtilis_. Whether the two are identical, I am unable to determine. Rattray (Rev. Cos., p. 47) refers to H. L. Smith's Type Slide No. 100, from rice-field mud, Savannah, Ga., as _C. subtilis_. In Smith's slide, in my possession, a number of the forms show faint outlines of the large apiculi and are otherwise exactly like C. polyacanthus.