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Show 1885.] MISS B. LINDSAY ON THE AVIAN STERNUM. 697 lateral processes, and for a crescent-shaped ridge at the end. The former character is easily understood if we regard these processes as homologous with those of the Ratitae, and consider the metasternum to have received special elongation at a late date. Plate XLIV. fig. 9 shows a condition which supports this view; the median part of the metasternum is here not longer than the process, while in the adult it is considerably longer; also the process is continued anteriorly into a ridge forming part of the undoubtedly costal sternum, which may be distinguished from the thinner cartilage of the metasternum adjoining it towards the median line. The crescent-shaped ridge marks the posterior insertion of the pectoralis minor, which thus lies as it were in a depression of the sternum ; this muscle is remarkable for its strength and its great elongation, taken in comparison with those of the pectoralis major. (iii.) THE GULL. (11 specimens, from about 16 days' to 5 days' embryos.) 1. In several individuals of about 14 days there was traced a thin ventral portion of the rectus continuous with the pectoralis major. 2. Two specimens of about 11 or 12 days exhibited traces, consisting in greater thickness and greater strength of the fibres in the regions indicated, of a division of the rectus into three bands, a median, and two lateral attached to the posterior-lateral processes. In the adult the lateral part of the rectus is, on the contrary, very thin. The embryonic condition recalls that of the Ostrich ; possibly the width of the sternum, which is somewhat broad for its length, may be associated with the early lateral thickness of the rectus. 3. The intercostales externi exhibit in the later stages the changes already described in the Guillemot; the supercostal sheet at the stage in which it is continuous sends a thin continuation under the pectoralis major, which passes over the sternum and is attached to it near the borders of the pectoralis minor. A 5 days' embryo affords the clue to the intercostal nature of the primitive transverse bands seen in the previous types ; bands precisely similar are seen alternating with and attached to the ribs, not overlying them as in the former cases ; while part of their dorsal extent is already fused to form the continuous supercostal sheet referred to above. The connecting link between the supercostal and intercostal state of these bands is seen in the Chick, where the bands overlie the ribs, but where they are found on dissection to alternate with them in the cervical region. 4. The metasternum has two pairs of processes: of these the outer is identified as homologous with the posterior-lateral process in the Guillemot and Ostrich by the fact that its outer border affords the sternal attachment of the obliquus externus. The inner one is therefore of later origin; perhaps it owes its existence to the peculiarity of the posterior end of the pectoralis major, of which the lateral part is unusually strong and longer than the median, and is attached to |