Shell Shape main content.

Shell Shape

Common shell shapes for metro area species  


A Black rock, matte finish.
Rhomboid Subtriangular
subelliptical rhomboid / subtriangular subtrapezoidal subovate


Ecomplanatasil Small
Elliptio complanataeastern elliptio


Alheterodon Silhouette Left
Alasmidonta heterodondwarf 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.


Lcariosa Silhouette Small
Lampsilis cariosayellow lampmussel


L radiata Silhouette
Lampsilis radiataeastern 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.


Aimplicata Silhouette Small
Anodonta implicataalewife floater


Mmargaritifera Silhouette Small
Margaritifera margaritiferaeastern 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).