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FOSSIL REPTILIA OF THE 



articular surfaces for the coracoid (figs. 7 and 8, c , d) is sub-triangular, convex 

 transversely, concave in the opposite direction, with the lower angle continued 

 down upon the side of the thickest part of this anterior portion of the sternum. 

 The back part of the articular surface rises higher than the front, so that the 

 general aspect of the surface is obliquely upward, forward, and outward. The 

 two surfaces are separated by a non-articular depression (d), of the breadth of one 

 coracoid surface ; this depression is bounded, like the sella turcica of the human 

 sphenoid, by a transverse rising or ridge of bone (fig. 7, a), continued between the 

 hinder angles of the two articular surfaces, and in front by the manubrial tubercle 

 (5), from which the upper border of the produced keel is continued. The ster- 

 num contracts behind the articular region at y, figs. 8 and 9, and then expands 

 rapidly in the horizontal direction, to form the broad, lamelliform body of the 

 bone (h), which, in Pterodactylus sucvicus* appears to have been almost semi- 

 circular in shape, and to have extended backward beneath about one half of the 

 thoracic abdominal cavity. The upper surface of the forepart of the sternal plate is 

 concave, and it becomes flatter as it expands. The lateral and lower surfaces are 

 also concave vertically, with linear markings, showing the implantation of the pec- 

 toral muscles that filled those concavities on each side the keel. Sufficient thickness 

 of the bone remains at the fractured posterior part (/), where the keel has not sub- 

 sided, to show the widely cancellous, and seemingly pneumatic, texture of the bone. 



The similar, but smaller and more mutilated, portion of a sternum of a Ptero- 

 dactyle (Tab. II, figs. 10 — 12) shows the same form and position of the cora- 

 coid articular surfaces, the non-articular intermediate depression, the lateral 

 emarginations or contraction of the sternum behind the part supporting the cora- 

 coids, and the backward extension of the keel beneath a certain proportion of the 

 expanded body of the sternum, forming the hollows for the lodgement of the 

 pectoral muscles. 



A sternum of the shape and proportions above described plainly indicates 

 pectoral muscles of great bulk and strength, by the extent of origin it afforded to 

 them, and by the depth of the depressions they filled on each side of the keel ; 

 but to what purpose the limbs moved by those muscles were put is best inferred 

 from the characters of the bone into which they were inserted. If, however, the 

 peculiar development of the fore limbs of the Pterodactyle had not been known, 

 the evidence of a pneumatic or widely cancellous structure in the thicker forepart 

 of the breast-bone would have suggested a power of locomotion in its original pos- 

 sessor akin to that of the class to' the sternum of which that of the Pterodactyle 

 makes, upon the whole, the nearest approach. 



It is true that the sternum is broad and shield-shaped in the Apteryx and 

 other land-birds devoid of the power of flight ; but this form, together with the 



* Quenstxdt, op. cit. 



