Among birds the sternum is greatly developed. Broad and short in the Ostrich tribe, it is devoid of a keel; and therefore the keel, if present in a bird, is suggestive of flight. The keel is differently developed according to the mode of attachment of the several pectoral muscles which cover a bird's breast. In several water birds the keel is strongly developed in front, and dies away towards the hinder part of the sternum, as in the Cormorant and its allies. The sternum in German Pterodactyles is most nearly comparable to these birds. [Illustration: FIG. 36. COMPARISON OF THE STERNUM] In the Solenhofen Slate the sternum is fairly well preserved in many Ornithosaurs. It is relatively shorter than in birds, and is broader than long; but not very like the sternum of reptile or mammal in form. The keel is limited to the anterior part of the shield of the sternum, as in Merganser and the Cormorant, and is prolonged forward for some distance in advance of it. Von Meyer noticed the resemblance of this anterior process to the interclavicle of the Crocodile in position; but it is more like the keel of a bird's sternum, and is not a separate bone as in Reptiles. In Pterodactyles from the Cretaceous rocks, the side bones, called coracoids, are articulated to saddle-shaped surfaces at the hinder part of the base of this keel, which are parallel in Ornithocheirus, as in most birds, but overlap in Ornithodesmus, as in Herons and wading birds. [Illustration: FIG. 37. STERNUM IN ORNITHOCHEIRUS FROM THE CAMBRIDGE GREENSAND Showing the strong keel and the facets for the coracoid bones on its hinder border above the lateral constrictions]