TLEUKOCERA. 73 



broad revolving baud on all the whorls, some have several bands, and 

 others again have a capillary line visible on the inside only. Four are 

 dark purplish-green, the color being caused by the broad bands on 

 the inside. It is nearly allied to T. moriforme hei-ein described, but 

 is not cylindrical. The specimens are usually of a very regular pyra- 

 mid with a short base. The carina of the periphery is usually strong, 

 but not always so. In this it is near to Melania {Trypanostoma') filum 

 (nobis), but it is more slender than that species. The aperture is 

 about one-third the length of the shell. Most of the specimens are 

 slightly channelled on the lower whorl. I name it after L. B. Thorn- 

 ton, Esq., to whom I am indebted for many fine specimens of this and 

 other shells. — Lea. 



This species is shorter in the canal, possesses wider bands 

 and wants the tubercles of moniUferum which it otherwise 

 much resembles. 



17. P. trivittatum, Lea. 



Trypnnostoma trivittatum, Lea, Proc. Acad. Nat. Sci., p. 175, 18(j2. Jour. Acad. 

 Nat. Sci., V, pt. 3, p. 282, t. 36, f. 102. Obs., ix, p. 104. 



Description. — Shell smooth, subfusiform, rather thin, shining, oliva- 

 ceous, three-banded; spire conical, pointed, carinate at the apex; 

 sutures line-like; whorls, eight, flattened, the last one being large; 

 aperture rather large, rhombic, banded within; outer lip 

 acute, sinuous ; columella slightly thickened and incurved. 



Operculum ovate, dark brown, with the polar point near 

 the base. 



Habitat. — Tombigbee River, Mississippi; Wra. Spillmau, 

 M.D. 



Diameter, '39 ; length, "78 of an inch. 



Observations. — I have examined about twenty specimens of this 

 species and find them differing very slightly. Every one has three 

 bands, the lower two of Avhich are more distinct on the outside than 

 the upper one, while inside they are well defined and nuicli alike. 

 Three of the specimens are very dark, almost purple, but tlie bands 

 are distinguishable inside. There is a white line immediately below 

 the sutures. In some specimens there is a disposition to l)e somewhat 

 angular on the periphery, below which there are transverse strire in 

 some individuals. The aperture is about three-eighths the length of 

 the shell. — Lea. 



