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Show 1040 MR. W. P. PYCRAFT ON THE [Dec. 19, Vicq-d'Azyr (20), Owen (13), and others to be the homologue of the olecranon, is considered by Shufeldt as a sesamoid only Prof. D'Arcy Thompson (19) sees, apparently, like Selenka (IS) and Flourens/in the cnemial crest of the Grebes^and Divers nothing more than " the upper extremity ot the tibia. In a preparation in our Museum Collection, of the pelvic limb of a nestling Grebe, the cnemial crest forms a part ot the tibial epiphysis in which a centre of ossification is just making its appearance (fig. 3, p. 1039). Prom this it would appear that the process in question is really only a greatly elongated epiphysis The fibula in the Colvmbi extends downwards to the level ol the superior border of the extensor bridge ; it terminates in the Podicipides near the distal ^ of the tibio-tarsus. The patella in the Grebe is a very large, laterally compressed pyramidal bone, the apex projecting above the level of the cnemial process, whilst its inner surface is more or less closely applied to the outer border of this process. Its base forms a longitudinally elongated glenoid surface for articulation with the femur. In all the skeletons of Colymbi in the Museum Collection, unfortunately, the patella is missing. According to Shufeldt (18) and others it is, however, represented by a small flake-like bone. Prof. D'Arcy Thompson (19) holds that the patella proper of the Divers has fused with the cnemial process, and that the small patelliform plate is to be regarded as a sesamoid, and not as the homologue of the free patella found in Podicipes, Hesperornis, aud other forms. This is a point which could probably be settled by an examination of nestlings or embryos. Amongst the Alcidae, e. g. Uria alle, the patella is more or less quadrate in form, and articulates by the lower half of its anterior surface with the apex of the cnemial process, this being very much less developed than that of the Grebe and Diver. Thus, the upper half of this border serves as a further extension dorsad of the process itself. From this it will be remarked that the position, size, and form of the patella, and the development of the cnemial process, in the Alcidae is distinctly different from that of the Colymbi, as also, it will be remembered, is the form of the pelvis. In all these particulars it will be noticed that, though there is a tendency in the Alcidae to modification along the same lines,- to a convergence of characters, due to similar methods of progression, resulting in a similar upright carriage when on land,-the Alcidae are less specialized than the Colymbidae, which possibly had its origin in that of the stock of the Cretaceous Hesperornis. The pelvic girdle and limb, in common with the rest of the skeleton, of this bird, are, as is well-kuown, almost indistinguishable from those or the modern Colymbi. Indeed, when we eliminate the presence of teeth, the Ratite sternum, vestigial wing, and complete ilio-ischiadic fissure, the only points of difference appear to be such as serve to distinguish species one from another. |