The Collaborative International Dictionary of English v.0.48:

Plagiostomi \Pla`gi*os"to*mi\, n. pl. [NL., from Gr. pla`gios
   slanting + ?, ?, mouth.] (Zool.)
   An order of fishes including the sharks and rays; -- called
   also Plagiostomata.
   [1913 Webster]