472 B I 



these characters : it is shortened, as in the birds of 



Erey, to admit a free motion of the legs, and the 

 iteral processes backward are formed into a sort of 

 strong, yet partially flexible arches, for defending the 

 sides of the bird. 



Eared Grebe. 



The structure of the shoulder is also well worthy 

 of attention. The form of the clavicle resembles that 

 of birds of prey, both in its curvature of opening and 

 its curvature downwards. The strength arising from 

 these curves is, however, reversed to that of the pre- 

 datory birds. In them the curve of the opening is 

 the more perfect arch, and the bone is flattened in the 

 direction of that arch ; whereas, in the grebes, the 

 lateral arch, which extends from the angle of the 

 sternal keel to the head of the scapular, is the better 

 formed, and the bone is flattened laterally to give 

 additional stiffness to that. The head of the clavicle 

 also bears against that of the scapular something in 

 the same manner that the arch of a bridge bears 

 against its abutment ; and the scapular, by becoming 

 narrow toward its termination, resists a thrust on the 

 head much better than it resists a pull. 



From this formation of the shoulder, it will be seen 

 that though it forms not an unsteady fulcrum for the 

 action of the wing in flight, it is also well calculated 

 for resisting a strain or pressure from the front ; and 

 from the posterior portion it may be seen that pro- 

 vision is made there for a peculiar action of the feet. 

 The sternal apparatus of the grebe is thus a very 

 typical one ; and when the small and elongated head 

 and neck, and the lobed toes, with the flattened tarsi, 

 turned outwards on their articulations, are taken into 

 consideration, one might almost arrive at a knowledge 

 of the habit of the bird without further investigation. 

 No air-bird, not even the most typical of the falcons, 

 presents the most powerful arch of its clavicle to the 

 air against which it flies, neither has it the extremities 

 of this arch resting on the angle of the keel and the 

 head of the scapula as abutments, as is the case in 

 this bird. Thus it is evident, from the structure of 

 the shoulder alone, and independently of that of the 

 foot, that the grebe is formed for making its way 

 through the water, and not merely along the surface, 

 though the resemblance of the sternum to that of 

 some of the ducks shows that it is also in part a 

 surface swimmer ; and this inference is rendered the 

 more certain by that resemblance being to the ducks 

 of the intermediate division, which have the swimming 

 habit and the diving one in nearly equal perfection. 



2. Sterna of the true Divers. In these birds, the 

 duck-like character which is traceable in the sterna 

 of the grebes disappears ; and while the sternal 



RD. 



apparatus presents a still more powerful arch in front, 

 the posterior part of it is continued so as to support 

 more completely the under part of the bird. 



The sternum is long and narrow, a little concave 

 on the upper side, and convex on the under ; the keel 

 well developed, higher in front than the breadth of 

 the side of the sternum, concave in front, and much 

 advanced at the angle ; the under side convex, but 

 less so than that of the sternum, and gradually 

 diminished to the rear, having the middle process, 

 which is broad and rounded, a little flexible. The 

 sides are concave, and have seven or eight ribs on 

 each ; and the lateral processes, which arc rather 

 slender, and have their terminations curved toward 

 the central one, have the appearance of two short 

 sternal ribs in addition. The anterior lateral pro- 

 cesses are large, and have their points directed upwards ; 

 and the coracoids, which are strong and short, have 

 their bases very much enlarged to the rear, and 

 with large processes projecting in the same direction 

 as the lateral ones. The clavicle is very much curved 

 downwards, with a tubercle at the middle, bearing on 

 the angle of the keel (which is more rounded than in 

 the grebes), but not articulated to it. The branches 

 of the clavicle are flattened laterally, rather slender 

 at the middle point, but much enlarged at their union 

 with the coracoids and the scapulars. They abut 

 against the latter bones by a head consisting of two 

 processes, and the coracoid has its enlarged head 

 applied over the junction of the two, and a process 

 underneath, which bears up the head of the clavicle. 

 The scapulars are rather slender, bent into a regular 

 curve, and tapering a little toward their points. The 

 following figure of the sternal apparatus of the red- 

 throated diver (Colymbus septentri(jnalis),'m the [tropor- 



Diver. 



tion of half the lineal dimensions of nature, and shown 

 obliquely, half on the under side, half on the profile, 

 will afford a better idea of these energetic careerers 

 through the water than could be obtained from two 

 separate views. 



On examining this figure, it will be seen that the 

 keel of the sternum, the clavicle, and the scapular of 

 this apparatus, form nearly one continued curve, 



