Before following the key, look here for an explanation of
1. Pteropod or heteropod?

Pteropod Heliconoides inflatus

Small spiral shell coiling to the left (sinistral). Body cannot fully retract into shell

Heteropod Atlanta selvagensis

Small spiral shell (up to 14 mm), laterally compressed and coiling to the right (dextral). Body can fully retract into shell
2. Which genus?
Shell and keel of aragonite (calcareous)
Shell of aragonite (calcareous) and keel of conchiolin (flexible, very transparent). Keel often missing. Spire consisting of up to 3.25 whorls
Small, almost spherical and highly striated (zigzag spiral rows) juvenile shell of aragonite (calcareous). Enclosed in adult whorl of conchiolin and conchiolin keel in adult form. Overall shell shape spherical and involute.
Spire has a light brown colour and 2-3 fine spiral lines. Restricted to the Atlantic Ocean.
Spire has no ornamentation. 
Spire tall, smooth, cone shaped and tilted relative to the plane of coiling. No clear ornamentation.
Large eyes, wide umbilicus. Spire is not strongly tilted.
Umbilicus narrow. Spire strongly tilted, sutures very shallow.
Tubercules scattered on spire surface.
Tubercules in rows on spire surface.
Conical orange-brown spire with clear ornamentation. 
in production
Spire tall, but conical with an apical angle of 35.9–45.8°
Spire tall, slender and turret like.
Highly ornamented spire with 4 to 4¼ whorls, sometimes tilted and a red-brown colour.
Moderately ornamented spire with 3½ whorls. Red colour keel base and sutures.
Small, 3 mm max. 3¼ to 3½ whorls in spire. Keel base yellow. 
Large up to 12 mm. 3½ to 3¾ whorls in spire. Keel yellow-brown. No ornamentation.
First and second whorls ornamented with elevated fringes. 4¼ to 4½ whorls in spire.
Thin shell wall. Tall keel that separates final whorls, large eyes and violet sutures.
Spire ornamented with numerous prominent spiral lines. 4½ to 4¾ whorls. Eyes type C.
Spire ornamented with numerous, interrupted spiral lines. 3½ to 3¾ whorls. Eyes type A.
Atlantic Ocean.
Pacific and Indian oceans.
Spire sutures shallow giving spire a globular appearance.
Spire sutures deep.
Spire surface covered with numerous low projections. Only found between 37-48°S
Only found off the west coast of America.
Spire sutures shallow giving spire a globular appearance.