The Collaborative International Dictionary of English v.0.48:

Selachii \Se*la"chi*i\, n. pl. [NL., fr. Gr. ??? a fish having
   cartilages instead of bones.] (Zool.)
   An order of elasmobranchs including the sharks and rays; the
   Plagiostomi. Called also Selacha, Selache, and
   Selachoidei.
   [1913 Webster]