ornithoscelida

from The Century Dictionary.

• A remarkable order of extinct reptiles presenting many characters intermediate between those of Reptilia and Aves, the ornithic modification being especially well marked in the pelvic arch and limb, whence the name.

from the GNU version of the Collaborative International Dictionary of English.

• noun plural (Zoöl.) A group of extinct Reptilia, intermediate in structure (especially with regard to the pelvis) between reptiles and birds.