﻿82 r.AMPSII.IS 



Var. gibbo Simpson. 



Shell considerably shorter and more solid than the type, with 

 a decided, curved posterior ridgi^e, the dorsal region much 

 rounde<l or humped ; surface having strong, low, concentric 

 ridges ; muscle scars smaller than in the typical shell ; female 

 shell somewhat produced at post-1 )ase. 



Length 95, height 70, diam. 40 mm. 



Ohio River, and southward. 

 Unio crossus Sowert-.y. Conch. Icon.. X\'T. iSfuS, pi. xcv, fig. 



520. 

 Lampsilis ligamentinus var. gibbus Simtson, Syn., 1900, p. 540. 



Var. nigresccns n. v. 



Shell much elongated, subelliptical to subrhomboid ; epider- 

 nmis dark brown to 'blackish, almost or quite destitute of rays. 



Length 115, height 61.5, diam. 44 mm. 



Length 113, height 60, diam. 38 mm. 



Allegheny County, Pennsylvania; Mississippi River, Le.' 

 County, Iowa ; Kankakee River, Illinois. 



A greatly elongated form with very dark, almost or quite 

 rayless epidermis. It seems to be sufficiently distinct to be 

 worthy of a varietal name. 



Lampsilis ligarnentina is an abundant, widely distributed and 

 variable form, generally of large size, and often quite ponder- 

 ous. There are, however, occasional shells of considerable size 

 and light structure. South of the Ohio River and west into 

 southern Missouri and Arkansas the type is generally replaced 

 by the variety gibba, which in many cases is distinct enough 

 to be entitled to specific rank, but the two forms are connected 

 by manv intermediates. Some specimens of the variety ap- 

 proach L. orbiculata in form, but it is a rougher shell, greener 

 colored, and usually has strong, low, concentric ridges. 



The name crassus was applied to this species by Say before 

 that of Lamarck was given to it. but Retzius had previousl)r 

 used the same name for an European Unio. 



