Introduction

Atlanta helicinoidea is a small species, with a maximal shell diameter of 2 mm. The last whorl in adult shells is broadly oval to rounded. The keel is moderately tall and rounded in undamaged specimens, with a posteriorly-slanted corrugated texture. The spire is low conical, consisting of 4-1/2 to 4-3/4 whorls. The second through fourth whorls have spiral ridges that increase from two in the second whorl to four in the first half of the fifth whorl. Spire coloration is dimorphic; light purple or light yellowish tan, due to underlying tissue pigmentation. Eyes type c, with a black cup-shaped base (unique in the genus), operculum type c, and radula type I. The species is cosmopolitan in tropical to subtropical waters, and is limited to the upper 100 m of the water column in Hawaiian waters. The species shares many similarities with A. inflata, but is immediately distinguishable by its eye morphology.

Diagnosis

• Shell diameter <2.0 mm
• Keel moderately tall and rounded, with a posteriorly-directed corrugated texture
  
• Spire low conical, consisting of 4-1/2 to 4-3/4 whorls
• Spire whorls with prominent spiral ridge sculpture
  
• Spire coloration either light purple or light yellow to yellow-tan
• Eyes type c, with a black, cup-shaped base (unique in the genus Atlanta)
  
• Operculum type c, radula type I