| OCR Text |
Show 1899.j OSTEOLOGY OF THE PYGOPODES. 1041 Ihe form of the patella and cnemial process of the Alcidae more nearly resembles that of the Impennes, as does, to a lesser extent, the pelvis. The form of the tarso-metatarsus in the Grebes and Divers is very similar. That of the Grebe may be distinguished from the Diver by tbe larger size of the intercotylar tubercle and the great depth of the inner glenoid surface for the inner tibio-tarsal condyle. In both groups tbe tarso-metatarsus is much compressed laterally, and the ectotrochlea is much reduced. The hypotarsus is simple. Other characters will be found in the Key (p. 1044). The phalanges of the toes are much flattened dorso-ventraily, the ungual phalanx especially so. The 4th digit is longer than the 3rd. x. SUMMARY. The present paper affords good evidence in favour of the views of Beddard, Eiirbringer, Gadow, and others who hold that the Grebes and Divers are closely related, but refuse to associate them with the Auks and Gulls as was done by Huxley aud others. The Pygopodes (=the Colymbi of Beddard) seem to be nearly related to the Tubinares, the Impennes, and the Steganopodes ; but, as Mr. Beddard remarks, " any comparisons bristle with difliculties." That Hesperornis rightly belongs to this sub-order there can no longer be any doubt, after Prof. D'Arcy Thompson's admirable memoir ; there is one point which has apparently escaped the notice of this writer, however, with regard to the pelvic girdle of Hesperornis. This differs from that of both Grebe and Diver, in that the pre- and postacetabular ilium form one great, vertical and laterally compressed blade of very considerable depth. In the Grebe and Diver the preacetabular ilium takes the form of a narrow blade, twisted so as to lie in an obliquely horizontal position. Eurthermore, Hesperornis seems to be peculiar in that the innominate bones meet throughout in the mid-dorsal line, above the neural crest of the synsacrum; in this particular, however, it approaches the Grebes, where the postacetabular ilium behaves in this way ; similarly it agrees with the Grebes in the shortening of the sternum and the large size of the patella. These last two points, however, must be regarded as coincidences rather than indications of affinity ; that is to say, Hesperornis must not on account of these points be regarded as more closely allied to the Grebes than to the Divers. Indeed, its sternum differs materially from that of both these families in that it was keelless, whilst the patella differs from that of the Grebe in being pierced by a foramen for the ambiens. But these and other points will be found exhaustively discussed in the memoirs of Marsh and D'Arcy Thompson. Mr. Beddard regards the Grebes and Divers as representing two |
