In its narrower sense, meaning the study of mollusk shells, conchology does not only include the study of marine mollusks (see seashells), but also the study of the shells of land and freshwater mollusks. As well as studying the shell, in some gastropods, the operculum, where present, is also studied.
Not all shell-collectors are conchologists: some collectors are primarily concerned with the perceived beauty (aesthetic value) and the extreme variability of shape, color and pattern in shells, as opposed to being interested in systematics or other scientific study of these natural history objects.
It is also true that not all conchologists are shell-collectors: this type of research only requires access to private and/or institutional shell collections. The terms shell-collector and conchologist can be regarded as two distinct categories, although there is some debate in the conchological community, with some people considering that all shell-collectors, regardless of their motivation, are conchologists of one kind or another.
Conchology can also be viewed as one aspect of malacology: the study of molluscs (UK spelling) or mollusks (US spelling). Malacology studies mollusks as whole organisms, not just their shells. Those who study malacology are known as malacologists. Conchology predated malacology as a field of study by many years: ever since at least the time of the ancient Egyptians (reference needed), people have collected mollusk shells.
In current times, conchology (in the more constrained sense) is often seen as rather archaic study: scientifically speaking it can be considered to be lacking in thoroughness, because of the limitations of looking only at the shell of an organism.
In the scientific or natural history sense, conchologists study shells as one aspect of the animal. The shell can give at least some insight into the diverse and complex taxonomy of mollusks. In older natural history collections, going back to the time of Linnaeus and before, usually the shell of mollusks from distant localities was the only part of the animal that was available for study. Even in current museum collections, it is quite commonly the case that the amount of dry material in the collection greatly exceeds the amount of preserved alcohol material.
Conchology deals with all mollusk shells, however, squid and other cephalopods do not have outer shells (with the exception of the Nautiloidea), having evolved to have only an internal bone or shell, used for buoyancy or support. Some groups (such as the aptly named nudibranchs) have lost their "skeleton" (internal and/or external) altogether, while in some it has been replaced by a proteinaceous support structure. Because of this, conchologists deal mainly with four molluscan orders: the gastropods (snails), bivalves (clams), Polyplacophora (chitons) and Scaphopoda (tusk shells).
There have been seashell necklaces found from the Stone Age, some of which were found in areas removed from the ocean, indicating that they were traded. Shell necklaces and jewelry are found at almost all archaeological sites, including at ancient Aztec ruins, digs in ancient China, the Indus Valley, and Native American sites.
During the Renaissance, people began taking interest in natural objects of beauty to put in wunderkammern. Because of their attractiveness, variety, durability and ubiquity (shell-bearing molluscs can be found from nearly all marine habitats and a huge variety of land and fresh water areas), shells became a large part of these collections. Towards the end of the 17th century, people began looking at shells with scientific interest. Lister in 1685-1692 published Historia Conchyliorum, which was the first comprehensive conchological book, with over 1000 engraved plates.
George Eberhard Rumpf, or Rumphius, (1627-1702) was another important early conchologist. He published the first classifications of molluscs into different groups; he suggested "Single Shelled Ones" (Polyplacophora, limpets, and abalones), "Snails or Whelks" (Gastropods), and "Two-Shelled Ones" (Bivalves). Rumphius first published many of the names and taxonomic terms adopted by Linnæus, and continued to do important scientific work even after he went blind, working by feel.
The study of shells & molluscs, like most other branches of zoology, was revolutionized by the "father of modern taxonomy" Swedish naturalist Carolus Linnæus and his system of nomenclature. It is now commonly held that 683 of the 4000 or so animal species he described are now considered to be mollusks (see Harry G. Lee's excellent article at the Jacksonville Shell Club website (which contains many well researched conchological articles) for details), although Linnaeus placed them in several phyla at the time.
After Linnæus, conchology/malacology became an official branch of zoology. There have been many prominent conchologists in the past few centuries; the Sowerby family were famous collectors and shell dealers, as well as being noted for their superb illustrations; John Mawe (1764 – 1829) produced arguably the first conchology how-to guide - The Voyager's Companion or Shell-Collector's Pilot as well as The Linnæan System of Conchology; Hugh Cuming (1791-1865) also is famous for his huge collection and number of new species discovered. Another fundamental work was American Conchology, or Descriptions of the Shells of North America, Illustrated From Coloured Figures From Original Drawings, Executed from Nature (six volumes, 1830-1834), written by Thomas Say.
Perhaps the most prominent conchologist of the 20th century was R. Tucker Abbott. Author of dozens of books on conchology, Senior Advisor, Founding Director, and finally Museum Director of the Bailey-Matthews Shell Museum, Abbott brought the world of conchology to the public. His most prominent works are "American Seashells" 1955 & 1974, Seashells of the World, 1962, and The Kingdom of the Seashell, 1972. See Conchologists and Guido Poppe for others. Many of the finest collections of seashells are in private hands. John du Pont, and Jack Lightbourne, among others, are known for extensive collections. Emperor Hirohito of Japan also amassed a huge collection, and was a competent and respected amateur conchologist. That said, John DuPont donated his shell collection to the Delaware Museum of Natural History (DMNH) in 1984, and by far the world's largest assemblage of mollusk shells is housed at the Smithsonian Institution, which has millions of lots and perhaps 50,000 species, versus perhaps 35,000 species for the largest private collections.