Shell Shape

Common shell shapes for metro area species  



Rhomboid Subtriangular



subelliptical rhomboid / subtriangular subtrapezoidal subovate


Ecomplanatasil Small

Elliptio complanata
eastern elliptio


Alheterodon Silhouette Left

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.


Lcariosa Silhouette Small

Lampsilis cariosa
yellow lampmussel


L radiata Silhouette

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.


Aimplicata Silhouette Small

Anodonta implicata
alewife floater


Mmargaritifera Silhouette Small

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).