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Diatoms of the United States is now known as Diatoms of North America.
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Glossary terms starting with "h"


The broader apex, or pole, of a clavate or cuneate valve. Example genera include Didymosphenia, Gomphoneis, Gomphonema, and Peronia.

Helictoglossa - terminal nodule, infundibulum, polar nodule

The internal, distal termination of the raphe in the shape of a pair of lips or a rolled tongue. The helictoglossa occurs in many raphid diatoms. If present, it can be distinguished in the light microscope in valve view by optical dissection (focusing through many narrow, optical planes). It may also be visible in girdle view as a thickened lip of silica. Plural form is helictoglossae. From Greek for rolled tongue.


A valve with poles, or apices, of the apical axis having different shapes. Heteropolar valves are asymmetrical to the transapical axis. Example genera include Gomphoneis and Gomphonema.

Heterotopic position

Applies to some concentrically undulate species of Stephanodiscus (e.g., S. aegyptiacus) where fultoportulae on the valve face are in the central area of the convex valve and in the marginal area of the concave valve. From Greek for “different place”.

See contrasting term, isotopic position.


Frustules in which one valve differs from the other morphologically. Frustules may be heterovalvar in their valve ornamentation or their raphe system. The term is often applied to the monoraphid diatoms, in which one valve has a normal raphe and the other is rapheless.



(1) The illustration, collection, or specimen that represents a taxon. (2) A type specimen. (3) A type material recognized by the International Code of Nomenclature for algae, fungi, and plants (ICN).

For diatoms, many authors designate an individual frustule or valve on a slide as the type specimen. They do this by imaging this specimen and circling it with a diamond objective marker, or marking its coordinates with a device such as an England Finder. While the ICN allows the whole slide containing the type specimen to be designated as the holotype, diatomists tend to avoid this practice in order to minimize later misinterpretation of the intent of the author.

The ICN uses a system called typification that makes objective identification possible by linking the name of a taxon with a holotype. Typification is intended to create a stable system of nomenclature by tying the published name to a holotype or a suite of holotypes.

Several other kinds of type material are recognized by the ICN, including isotype, lectotype, neotype and paratype. Types are not expected to be “typical” or “idealized” specimens, although workers may find such use of the terms.

See also synonym.

Hood - cavum

A silica extension with the shape of a hood on the inside of a valve. It occurs only in some species of Planothidium where it partly covers the inside of the asymmetrical central area of the rapheless valve. See also rimmed depression.


Any area of a diatom valve that is unornamented (lacking pores or other structures).


A very delicate, porous membrane of silica covering the opening of an areola or an alveolus. The pores of the hymen may be round or elongated and range from 5-10 nm (0.005-0.010 ┬Ám) in their shortest diameter. Found in many raphid genera such as Cocconeis and Neidiopsis. Visible only with the SEM. Plural form is hymenes.

A similar structure with larger pores, found in many centric and some raphid diatoms, is the cribrum.



The smaller theca of the frustule. The hypotheca is overlapped by the epitheca. From Greek for inner sheath.