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Shell Shape

Common shell shapes for metro area species  

 

Subelliptical

Rhomboid Subtriangular

Subtrapezoidal

Subovate

subelliptical rhomboid / subtriangular subtrapezoidal subovate

 

Ecomplanatasil Small

Elliptio complanata
eastern elliptio


subtrapezoidal 

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


subovate

L radiata Silhouette

Lampsilis radiata
eastern lampmussel


subovate

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


subelliptical

Mmargaritifera Silhouette Small

Margaritifera margaritifera
eastern pearlshell


subelliptical

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).

American Museum of Natural History

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