Molds and casts

This picture shows a specimen of the Eocene Turritella andersoni (height 7.5 cm) from southern California (please see one of my earlier posts if you want more information about this species). If the specimen is removed (i.e., is weathered away or simply fell out), it leaves behind an external mold = an impression of the external surface. This external mold is a negative surface, that is to say, it could literally "hold water." 

This next picture shows the external mold of the original specimen and, to the right, an external cast of this mold. Most collectors commonly do not bother to collect external molds. At some localities, however, that is all you can find, and if you need to identify the genus and species, it will help if you create a latex external cast of the external mold. That way, you can make a "positive" out of a "negative." All you need is some liquid latex, like the kind you can put on the back of a rug to keep it from sliding around on a floor. Carefully pour the liquid latex into the external mold (try not to create any bubbles), and let the latex dry. Removal of the latex cast is easy; just pull it off. A latex external cast is shown above just to the right of the external mold. In some, cases nature creates external casts by filling in the external mold with some foreign substance. In the above, because I used latex to make the external cast, therefore, it is called an artificial-external cast.

The above picture shows an internal cast, which shows the interior of a high-spired gastropod shell (most likely, a Turritella). The 6-cm high shell was hollow, and silt and mud filled the shell after the death of the gastropod. Later, the sediment converted into solid rock, the shell was destroyed, and all that is left is the internal cast. Equivalent terms for an internal cast are endocast, internal "mold," or, my personal favorite, steinkern (= a German word meaning a "rock center or core"). Steinkerns are not very useful for determining genus or species. Some early workers, unfortunately, used them for naming new species. Doing so has caused serious taxonomic (classification) problems for subsequent workers, who commonly refer to such a species as a nomen dubium = a name representing a species that is not identifiable from the original specimen (type) used to describe it.  

This final picture shows the external mold (6 cm wide) of the bivalve Laevicardium californiense. The fossil is of Plio-Pleistocene age and from the Santa Barbara Formation, Santa Barbara, southern California.