the port of determination is the linear ridge above mentioned, viz. the "ancono-deltoid ridge." It varies, in different birds, in its relative position to the pectoral ridge. In Raptores it is relatively closer to the base of that ridge than in Argillornis. In Pelecanus its relative position agrees better with that of the ancono-deltoid ridge in the fossil, but it inclines distally toward the radial border of the bone instead of from that border. In Diomedea both its course and relative position (fig. 13, k) at the part of the base of the pectoral ridge answering to that preserved in the fossil best agree therewith; but there is in Diomedea a second linear intermuscular ridge (ib. k') anconad of the first, of which there is no trace in the fossil.
But the sum of my comparisons of the present portion of humerus inclines me to see the nearest affinity to be to the longipennate natatorial or aquatic birds, and among them to the largest existing kind, viz. the albatross (Diomedea exulans), but with a difference of size indicated by the subjoined admeasurements:—
Argillornis. |
Diomedea. | ||||
|
12 | 9 | |||
A guiding modification of the distal portion of the humeral shaft in birds is the difference of form of the radial and ulnar sides of the bone. The radial side becomes, in most birds, the narrowest, as the shaft descends and expands toward the distal articular end; and in the albatross it is carried to the extreme of becoming a mere ridge for near 3 inches from that end (figs. 14, 15, l). The portion of the distal end of the shaft of the present fossil humerus, wanting the articular termination of the bone (figs. 10-12), presents this character, and, as in Diomedea, shows the narrow longitudinal groove on the palmar side of the ridge (fig. 11). This groove is the radial border of the triangular flat and shallow depression (figs. 11, 15, m) for the origin of the "brachialis anticus" muscle, which depression shows the usual linear roughness or sculpturing due to such relation of muscular attachment. The same sculpturing marks the triangular" prebrachial depression" (fig. 11, m) in Argillornis; but the triangle is longer and narrower in the fossil, and the free surface of the palmar part of the shaft, ulnad of the depression, rises more abruptly from it in Argillornis, and presents a more uniform transverse convexity than in Diomedea.
Unfortunately the characteristic ectepicondylar process (fig. 15, n) of Diomedea has been broken away, if it ever existed in Argillornis, with the rest of the distal end of the humerus in the present fossil (fig. 11); but it may be remarked that the presence of such a projecting part of the humerus, like the pectoral and subtuberous ridges in Diomedea (figs. 4 & 5, c' & i) would render the rolled fossil more liable to such fracture and loss.
In relation to other species and genera of birds based on fossil
