Introduction to molluscan taxonomy 2) The significance of types

It is very difficult to define or characterise a taxonomic entity solely by means of words. It is obvious that more secure ’standards' are required to tie scientific names firmly to their objectives. These standards are the types.

The modern type concept has developed slowly. The original draft of the International Rules of Zoological Nomenclature (1901) did not include any directives concerning types. Formal rules and recommendations regarding type specimens were adopted at the Paris Congress (1948).

The type of a species is a definite specimen, but the type of a genus is a designated species. It does not matter how many new taxonomic catagories and characters are discovered, the verbal definitions may be continuously modified and improved by reference to the types. Whenever types are available, it is an easy matter to check them for newly discovered taxonomic characters.

The original type specimen is the last court of appeal in cases of doubt as to the applicability of a name. If a description and a type specimen seem to apply to different entities, the name should be assigned to the species to which the type specimen belongs, provided it is certain that it is the type selected by the original describer.

Types of subspecies are subject to the same rules as types of species. The type of a species is always simultaneously the type of its nominate sub-species.

Kinds of Type Specimen

The principle kinds of types may be classified and defined as follows:

I. Primary types.

A. Holotype (or simply type). The single specimen designated or indic¬ated as 'the type' by the original author at the time of publication of the original description.

B. Paratype. A specimen other than the holotype which is before the author at the time of original description and which is designated as such or is clearly indicated as being a specimen upon which the original description was based.

C. Syntype (= cotype). One of several specimens on which an author bases an original description when no single specimen is designated as the holotype.

D. Lectotype. One of a series of syntypes which is selected subsequent to the original description and thenceforth serves as the definite type of the species. In order to be effective, such selection must be made known through publication.

II. Supplementary types.

A. Neotype. A specimen selected as type, subsequent to the original description in cases where the primary types are definitely known to be destroyed.

B. Plesiotype. A specimen on which subsequent descriptions or figures are based.

III. Typical specimens.

A. Topotype. A specimen not of the original type series collected at

the type locality.

B. Metatype. A specimen compared by the original author with the type

and determined by him to be conspecific with it.

C. Homotype. A specimen compared by another than the author of a

species with the type and determined by him to be conspecific with it.

The International Commission at its Paris meeting (1948) officially sanctioned the use of three kinds of types - holotypes, syntypes and lecto¬type s - as being "available to supplement the characters noted in the original description".

Actually most of this elaborate nomenclature of types is superfluous. As Williams (1940) has stated correctly, "There can be no possible reason for having any other type except a single one for each name". Only holotypes, syntypes, lectotypes and neotypes have nomenclatural significance, but some of the additional terms for types mentioned above have proved useful on zoological grounds.

Further Reading

Mayr, E., Linsley, E. G., and Usinger, R. L., 1953» Methods and Principles of Systematic Zoology. McGraw-Hill, New York.

Simpson, G. G., 1940, Types in Modern Taxonomy. Amer. Journ. Sci., 238, 413-431.

Williams, C. B., 1940, On type specimens. Ann. Ent. Soc. Amer., 33, 621-624.