﻿TRUNCILLA 1 3 



at the rest lines. It is thinner than the rest of the shell and 

 much excavated within. Both male and female shells often 

 have a shallow, radial furrow above the posterior ridge. 



Length (male) 64, height 47, diam. 43 mm. 



Length (female) 65, height 42, diam. 58 mm. 



Length (female) 57, height 39, diam. 38 mm. 



Length (female) 56, height 38, diam. 45 mm. 



Tennessee and Cumberland River systems. 



Type locality, Tennessee River. 

 Unio arcccfonnis Lka, Tr. Am. Phil. Soc, IV, 1831, p. 116, 



pi. xvTi, fig. 44; Obs., I, T834, p. 126, pi. xvii, fig. 44.— Des- 



HAYES, Tr. Element. Conch., 1839, p. 18, pi. xxix, fig. 6.— 



Hanley, Biv. Shells, 1843, p. 182, pi. xxii, fig. 40. — Des- 



HAYES, Traite Element. II. 1853. pi. xxiv, fig. 6.— Chenu, 



111. Conch., 1858, pi. XII, figs. 3, 3a, 3&.— Reeve, Conch. 



Icon., XVI, 1864, pi. XIV, fig. 57. 

 Margarita (Unio) arcccfonnis Lea, Syn., 1836, p. 18 ; 1838, 



p. 16. 

 Margaron (Unio) arcccfonnis Lea, Syn., 1852, p. 2}, ; 1870, 



p. 36. 

 Truncilla arcccformis Agassiz, Arch, fiir Naturg., I, 1852, p. 



44. — Simpson, Syn., 1900, p. 519. 

 Unio nexus Say, Trans. Jl., IV, 183 1, p. 527; Am. Conch., 



Pt. 6, 1834, pi. LI.— Conrad. Monog., VIII, 1837, p. 68, pi. 



xxxviii, fig. I.— KusTER, Conch. Cab. Unio, 1854, p. 69, pi. 



XVII, fig. 2. 



This species is much inflated, very solid and generally rhom- 

 boid or subquadrate in outline. The female shell when old is 

 more inflated than that of the male, the marsupial swellings 

 are greatly developed towards its base, but on the base of the 

 shell are flattened remarkably so that they do not project ma- 

 terially below the base line. The species does not seem to be 

 very closely related to any other member of the genus. Ac- 

 cording to Binney, Unio nexus was first published in Decem- 

 ber, 1831, (Bib. N. Am. Conch., I, p. 266), and Scudder states 

 that the fourth volume of the Transactions, containing Lea's 

 description of Unio arcccformis, was published at the latter 



