The Collaborative International Dictionary of English v.0.48:

Ectostosis \Ec`tos*to"sis\, n. [NL. See Ect-, and Ostosis.]
   (Physiol.)
   A process of bone formation in which ossification takes place
   in the perichondrium and either surrounds or gradually
   replaces the cartilage.
   [1913 Webster]