554 UNITED STATES GEOLOGICAL SURVEY OP THE TERRITORIES. 



intier lip very slightly thickened and reflected below, extremely thin above ; 

 outer lip sharp ; columella imperforate, and somewhat twisted. 



Length (of a medium-sized specimen), 0.55 inch; breadth, 0.26 inch; 

 length of aperture, 0.25 inch; breadth of same, 0.12 inch; apical angle 

 convex, divergence varying with age, from about 38° to 45°. 



This species bears some general resemblance to Bulimus Floridanus of 

 Conrad (Am. Jour. Conch., I, 144, pi. 11, fig. 11), from the Upper Eocene 

 at Tampa Bay ; but it is less elongated, and has a less elevated 'spire, and 

 less angular aperture than Mr. Conrad's species, while it also differs in its 

 smoother surface. 



I have seen a recent shell much more nearly resembling our species, 

 figured under the name Spiraxis limnceiformis, Shuttleworth ; but it has a 

 more ventricose body-volution and proportionally longer aperture than our 

 species, and differs in having its columella twisted in the form of a prominent 

 fold. 



Locality and position. — Opposite the mouth of Yellowstone River, North- 

 western Dakota ; from the Fort Union Lignite group ; probably Eocene. 



Genus COLTJMNA, Perry. 



Synon.— Columna, Perry (1811), Conch., pi. 51*;— Sebum. (1817), Ess., 188— Moller (1832), in Oken's Isis, 



129.— Beck (1837), Iud.,76.— H. and A. Adams (1855), Genera Recent Moll., II, 134 — 



Tryon (1866), Am. Jour. Conch., II, 297.— Binney (1869), Land and Fresh-water Shells 



of North America, part 1, 189. 



Acicula (lea Aiguilles), Blainv. (1825), Malac., 456 (as a section of Aehatina); not Acicula of 



Hartmanu or Risso. 

 Shodea, H. and A. Adams (1855), Genera Recent Moll., II, 135 (as a subgenus under Columna).-— 

 Biuney (1869), Land and Fresh-water Shells of North America, part 1, 190. 



Etym. — Columna, a pillar or column. 

 Type. — Aehatina columna, Muller.t 



Shell much elongated, cylindrical or terete, dextral or sinistral ; surface 

 nearly smooth, or decussately granular; apex obtuse; volutions convex or 

 flattened, lower ones generally constricted at the suture, last sometimes angu- 

 lar below; aperture auriculiform, comparatively small; columella not prop- 

 erly perforated, but spirally twisted so as to form an open canal all the way 

 up the spire, and abruptly truncated at the base; lip simple, straight, and 

 acute. 



* Perry's Conchology is not paged, and consequently I cannot cite the particular page on which 

 his description occurs. 



t Perry's type was Aehatina columna, Miiller, which is, I believe, considered a Eynonym of C. 

 flammca, Marty n. 



