Saurischian pelvis (left side)
Saurischia is one of the two orders of dinosaurs. In 1888, Harry Seeley classified dinosaurs into two main orders. Their hip structure was why they were put into these orders. Saurischians ('lizard-hipped') and the ornithischians ('bird-hipped') have differences in the ways bones in the hip are put together.
All carnivorous dinosaurs (the theropods) are saurischians, and so are the sauropods like Diplodocus and Brachiosaurus.
Birds are direct descendants of a group of theropod dinosaurs, so they are a sub-clade of saurischian dinosaurs in modern classification.