planispiral
/ˌpleɪnɪˈspʌɪrəl/ /ˌpleɪnɪˈspʌɪr(ə)l/adjective
Forming a flat coil; of or relating to a flat spiral form, as the shell of certain molluscs (including many fossil species), the proboscis of a butterfly or moth, etc.
Origin
Late 19th century; earliest use found in The Century Dictionary. From plani- + spiral adverb.