The Collaborative International Dictionary of English v.0.48:

Conchylaceous \Con`chy*la"ceous\, Conchyliaceous
\Con*chyl`i*a"ceous\, a. [L. conchylium shell, Gr. ?, dim. of ?,
   equiv. to ?. See Conch.]
   Of or pertaining to shells; resembling a shell; as,
   conchyliaceous impressions. --Kirwan.
   [1913 Webster] Conchyliologist