OCR Text |
Show 902 DR. R. W. SHUFELDT ON THE OSTEOLOGY OF THE [Dec. J, studies of the structure of the present genus. In T. alexandn we find that the bone has a round anterior margin with complete abortion of the manubrial process. Just within this line, the sternum is much thickened, mounting up above in the median line into quite a prominent pyramidal elevation. On either side of this a circumscribed pit is found, while mesially, and directly behind it, is a single pneumatic foramen. For the rest, the dorsal aspect of the bone is smooth and generally concaved. The costal processes are conspicuous, being directed forwards and outwards, and either costal border behind them supports five articulations for the costal ribs. Plate L X . fig. 6 shows the wonderful depth of the sternal keel in this unrivalled little prince of volants. In front it is fortified by a thickened rib of bone, as in most carinate birds, while its convex inferior margin is finished off with a rim, which appreciably projects beyond it all along its sides. The posterior sternal margin is entire, being convex outwards and approximately a portion of an arc of a circle with a radius of 6 millimetres (2*4 rams, in the fig., it being x 4). M u c h stress has been laid upon this fact in comparing the Trochi-lida with the Swifts ; but a comparison of the figures will at once show how essentially different these bones are. Turning now to the shoulder-girdle of Trochilus alexandri, w e are confronted with an exceedingly interesting structure and one which markedly departs from these elements in ordinary birds. Of the three bones the most striking difference is seen in the coracoid (Plate L X . fig. 5). This element has a straight shaft with a very slightly dilated sternal end, the whole being much compressed in the anteroposterior direction. Just above its middle it is pierced by an elliptical foramen with its long axis corresponding with the long axis of the shaft ; immediately above this, again, we find a similar foramen that represents the tendinal canal in other birds, but here completely surrounded by bone. The glenoid cavity is comparatively large and projecting, while the summit of the bene, when the arch is in situ, points towards the median plane. Professor Owen's figure of T. pella referred to above shows very well indeed the position of the furcula, when articulated as in life, in the Hummers. Its seemingly high position is largely due, however, to the great depth of the carina in these birds. As for the bone itself, w e find it assuming a form at the very limit of the U-shaped variety. Its limbs are almost filamentous in character, and the hypo-cleidium of rudimentary development only. Its heads are compressed from side to side, but very slightly enlarged, and are quite sharply crooked downwards and backwards to have their apices meet the scapulae. They rest against the coracoids on the mesial side of the bridge that closes in the tendinal canal alluded to above. A scapula contributes but a very small share of the articular surface to the glenoid cavity-less than a fourth, I should judge. |