Sternum. Pl. XII., fig. 1, and XIII., figs. 1, 2.
This bone is almost perfect, having lost only a few lines of its inferior margin, and, though it differs considerably from the characters attributed to it by Professor Owen, this is, without doubt, due to his not having had a connected fragment of the superior portion of the bone, so that its enormous posterior concavity was not appreciated. It is chiefly remarkable for its regular oblong shape, without any irregularities of outline or unossified interspaces. The texture must be cancellated, for, though apparently thick and massive, it is very light in proportion to its size, as will be seen in the appended table. Its general form is scaphoid, the concavity being very marked in the upper half, amounting to one inch in depth measured from a transverse chord, and to one-and-a-half inches in depth if measured from a longitudinal chord; the total length of the latter being 7 inches. The anterior width at the costal processes (a) is 4 inches, and at the posterior end of the costal border 3 inches 6 lines.
The costal border (e-e′) occupies half the lateral margin of the bone, the posterior half of the line being only slightly concave interiorly, and exteriorly being flat in the middle and sloping very slightly to the inferior angles. The superior margin is thin, and presents a wide mesial notch (f) and two lateral notches (g), which are bounded exteriorly by the costal processes, which project backwards and upwards for 6 lines. The coracoid grooves (b′) are 1 inch 6 lines in length and 2 lines in depth of anterior border. They are separated by a slight triangular interspace (f′) 5 lines wide, beneath which is a smaller triangular pit (h). The keel (c) commences by two angular ridges bounding this pit posteriorly, and forms a blunt process 3 inches in length (c-c′), expanded anteriorly to a rough tuberculate surface 4 lines in width and 9 in length, and then compressed into a narrow tuberculate ridge that is gradually lost in the smooth convex surface of the bone at less than one-half the distance from its superior margin. The greatest elevation of the keel above the convex surface of the bone is less than 3 lines. The impression (d) for the attachment of the pectoral muscle extends from the exterior angle of the coracoid notch towards the posterior part of the keel, including a triangular area which occupies only one-sixth of the exterior surface of the bone, showing the extremely limited and feeble attachment of the great muscles of flight. Large pneumatic foramina (i) exist in the interior of the bone at the upper angles, and one (i′) on the exterior surface on the left side only.