Shell Shape

Common shell shapes for metro area species  



Rhomboid Subtriangular



subelliptical rhomboid / subtriangular subtrapezoidal subovate


Elliptio complanataeastern elliptio

Elliptio complanata
eastern elliptio


Alasmidonta heterodondwarf wedgemussel

Alasmidonta heterodon
dwarf wedgemussel

rhomboidal to subtriangular

Shell shape is among the external features most useful for identification. The types of shapes are described by their resemblance to geometric figures. These descriptions are broadly accurate, although the shape of a species may vary, and some species are more plastic in their shape than others.

It is often useful to examine a series of shells or specimens in order to accurately gauge a species' range of variation.


Lampsilis cariosayellow lampmussel

Lampsilis cariosa
yellow lampmussel


Lampsilis radiataeastern lampmussel

Lampsilis radiata
eastern lampmussel


A term such as "sub-ovate" may apply to species that share variations of this general shape. Thus L. cariosa and L. radiata (left) share an ovoid form that is modified by a distinct "flattening" of the posterior shell margin- hence "sub-ovate", an oval shape appearing to be flattened on one side.

Note that a "triangular" shape, as in A. heterodon (above), refers to the slope of the shell margin on the anterior side.


Anodonta implicataalewife floater

Anodonta implicata
alewife floater


Margaritifera margaritiferaeastern pearlshell

Margaritifera margaritifera
eastern pearlshell


Compare the modification and variation of the elliptical shape in A. implicata and M. margaritifera (left) with that of the subovate shape in L. radiata (above). Also carefully compare the sub-elliptical shape of M. margaritifera with the rhomboid shape of A. heterodon (above).