Propeller shells and taxonomic notes

One of the missions of any natural history museum is to catalog its collections. Each specimen needs to be properly identified so it can be recorded and thus available to researchers in search of that particular species, or mineral, or trace fossil, etc. This is a continuing task. As new knowledge emerges, identifications may change.

The Bruce Museum has a large collection of mollusk shells that were transferred from the Smithsonian many years ago. This shell belonged to a propeller ark, so named because the twisted shell calls to mind a propeller. Propeller arks are bivalves that lie half buried in the sediment, with the twisted end exposed. Like common clams, they are filter feeders. I was struck by this shell because of its tiny, beautifully handwritten label with the delightful scientific name Parallelipipedum semitortum. Unfortunately, this name is no longer recognized - the proper name for the shell is Trisidos tortuosa.  

The tongue-twisting genus name Parallelipipedum is no longer considered valid by malacologists, as it was coined after Trisidos and thus lacks priority. All this is now duly updated in our catalog. The shell now returns to the collections, having spent a day visiting us upstairs, and awaits its next call, perhaps by a scientists, perhaps for an exhibition. For the record, the shell sits on a shelf adjacent on to a close relative, Trisidos semitorta (the half-propeller ark). As you might guess, that species has a less twisted shell.

Thanks to our science volunteer of the year, Hank Silverstein, for identifying this (and thousands of other) shells and data specialist Ann Burns for continually updating the catalog.