708 UNIO 
Unio tetralasmus var. manubus SIMPSON, Syn., 1900, p. 740. 
I am not aware that this form has ever been figured, but the 
I,ea collection has a right valve from Chihuahua, Mexico, 
which was donated by Dr. Gould under the name Unio man- 
ubius. ‘The form, while slightly rhomboid by reason of the 
oblique truncation of the posterior slope, becomes more nearly 
elliptical than the type. The epidermis is sometimes more 
nearly elliptical than the type. The epidermis is sometimes 
dark and may either be dull or shining. 
Var. dechivis Say. 
Shell of moderate size. subrhomboid, somewhat rounded on 
the base line; posterior ridge high, narrowly rounded and 
standing out somewhat distinctly from the rest of the shell; 
dorsa! slope having a wide, shallow, radial furrow just above” 
and reaching to the posterior ridge, decidedly obliquely trun- 
cated: surface dark brown, unevenly suleated and rough; 
nacre sometimes purple-tinted. 
Length of Say’s figure 74, height 39, diam. 25 mm. 
Lower part of the Gulf States from Alabama to Louisiana. 
Type locality, Bayou Teche, La. 
Unio declivis Say, Transylvania Jl. IV, 1831, p. 527; Am. 
Conch., III, 1832, pl. xxxv.—Han ey, Biv. Shells, 1843, p 
200, pl. xxi, fig. 50—CueENnu, Bib. Conch., Ist. ser., IO ie 
1645) p. AO, Ol) MINE, Mes cect. uae To Kuster, Conch. Cab: 
Unio, 1852, p. 60, pl. xiv, fig. 1—-Frrerson, Naut., XVII, 
1903, p. 49, pl. 1, lower fig. 
Margarita (Unio) declivis Lea, Syn., 1836, p. 32; 1838, p. 22. 
Margaron (Unio) declivis Lua, Syn., 1852, p. 33; 1870, p. 54- 
Unio tetralasmus var. declivis StMpSON, Syn., 1900, p. 740. 
Quite a distinct form, when typical, and apparently not very 
abundant. The posterior ridge is elevated, its edges being 
rather distinctly marked above and below ; it is rounded and 
rather narrow and ends behind above the base in a sort of 
beak, as noticed by Say. But I have seen specimens much 
