*Diatoms of the United States* is now known as *Diatoms of North America*.

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Stria density is based on the number of striae in an arc of circumference. The density = total number of striae on the valve * 10 / circumference. In Figure 1, the stria density is equal to (21) (10) / 54.3 µm, or 3.9 striae in 10 µm of circumference.

Stria count is based on the number of striae in a chord of the valve face. In Figure 2, the chord crosses 5 striae, giving a count of 5 in 10 µm.

**Anonymous. (1975).** Proposals for standardization of diatom terminology and diagnoses.
*Nova Hedwigia, Beihefte 53: 323-354.*

**Collins, G.B. and Groetsch, C.W. (1981).** An elementary mathematical problem arising in diatom taxonomy.
*International Journal of Mathematical Education in Science and Technology 12(1): 121-124.*

**Genkal, S.L. (1977).** On counting of some taxonomically significant structural elements of valves in the diatom algae of the family Thalassiosiraceae Lebour emend. Hasle (Bacillariophyta).
*Botan. Zhurnal 62(6): 848-851.*

**Theriot, E. (1988).** An empirically based model of variation in rotational elements in centric diatoms with comments in ratios in phycology.
*Journal of Phycology 24(3): 400-407.*

*Image Credit: Loren Bahls*

Figure 1. This diatom has a total of 21 striae.

*Image Credit: Loren Bahls*

Figure 2. The red line shows a 10 µm chord. It crosses 5 striae, giving a stria count of 5 in 10 µm.