Species are described on the basis of one or more shells, the type series. In case of the latter situation, one specimen is selected holotype by its author(s); the other(s) thus become automatically the paratype(s). In modern works, the type series is completely listed and any shell not mentioned in the original publication cannot be regarded a type specimen.
This practice is ruled by the International Code on Zoological Nomenclature, and their Code can be found at http://iczn.org/code
, with its many detailed regulations and subsequent jurisdiction in its Bulletin.
For those of you who don’t care about all the juridical finesses, a practical example.
In 1901, S.I. da Costa described a new species as Drymaeus exoticus from “The hot country, Upper Magdalena River, Colombia”. As measurements he gave “Long. 23.5, diam. 11 mm”. Apart from the description and the type locality, there were no further remarks on this species. At that time, it wasn’t uncommon not to state explicitly on how many specimens a description was made.
In my 1979 review of genera, I listed the species under Drymaeus (Drymaeus), and the single specimen that I had found in the London museum as “holotype”. Upon my return to London, last September, I re-studied the specimen, which is accompanied by a label in da Costa’s handwriting. As you may see, the locality matches the one given in the original publication. The shell proved to be slightly bigger than da Costa’s measurements: shell height 24.8, diameter 12.2 mm. Still, I had little doubt to question the “holotype” labeling.
In December last year, when working in the Brussels museum, I found a shell labelled as “D. exoticus d.C. Bogota” in the Dautzenberg collection. Dautzenberg usually had added his own label, stating on which date he acquired specimens from whom. But in this case not. It is clearly a label in da Costa’s handwriting, and we know that he was a shell dealer. However, it is not clear when he sold this specimen to Dautzenberg, and whether it was already in his possession when da Costa described the species in 1901 or not. The Brussels shell is 24.3 mm shell height.
Although the Brussels specimen isn’t labeled as “type” and bears no collection date, it cannot be excluded that it was in da Costa’s possession when he described Drymaeus exoticus, and formed part of the type series. Thus my 1979 listing becomes a lectotype designation under Art. 74.6 ICZN.
By the way, notice the imprecise (and incorrect) locality on the Brussels label: “Bogota”. This was a habit in the 19th and early 20th century of many collectors and shell dealers, viz. to mention the capital of the country instead of the exact locality. It makes taxonomy often a puzzling occupation…