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Show 1885.] MISS B. LINDSAY ON THE AVIAN STERNUM. 71 1 Tabular Classification of parts of the Sternum A. Part common to Saurojjsida and Mammalia :- Costal sternum, arising in two bands; connected with sternal ribs in the adult, but often losing its connection with ribs which took part in its early formation. B. Part common to Ratita and Carinata, but seen to be wanting in early embryos of the former, never of the latter; it is thus seen in process of formation in the former, as indicated by its partly cartilaginous condition in the adult:-Metasternum. C. Part apparently common to both Ratitae and Carinatse, but really of different origin :- Anterior lateral process; seen added to the costal sternum in the Ostrich, formed by atrophy of anterior ribs in the Chick and Gannet. D. Part absent in Ratitae, but common to all Carinatee:- Keel; the median ventral outgrowth of B. E. Part common to some Ratitae (Ostrich, and indicated in Rhea) and the majority of the Carinatas :- Posterior lateral process. F. Parts variant in families and genera of Carinatae : - Accessory processes of Metasternum. Rostrum or anterior median sternal apophysis. Xiphoid ends of posterior processes. Variations of the Posterior Lateral Process. The posterior lateral process of the sternum, if the value above assigned to it be correct, ought to be found a constant feature of the sternum of birds. Comparison of types shows that this is the case : the birds that have a sternum without processes are few, the most familiar instances being the Crane and the Goose. It must not, however, be supposed that whenever we find a posterior lateral process, this represents the primitive process as seen in the Ratitae; tor example, we find in the Spheniscidse, &c. (cf. figures given by Watson, 'Challenger' Report on the Spheniscidse, and by Forbes, Report on the Tubiuares), variations in the shape of the posterior processes which suggest that they have been formed by completion of the growth of the sternum. In such cases, however, the primitive process must form a part of the new one. A secondary process exists in a large number of birds ; and the study of development has already shown that such an accessory process may be added either on the external side of the primitive posterolateral process, as in the Chick, or on the inner side, as in the Gull. In the Pheasants, as well as in the Gallinaceae, it is doubtless formed as in the Chick : in many other birds, for example in the Bustards, Curlews, Plovers, & c , its exact origin is at present unknown. It is worthy of remark that the accessory process, when present at all, is not conspicuous in birds of archaic types, while the large |
