OCR Text |
Show 44 DR. R. \y. SHUFELDT ON T H E [Jan. 18, glancing at the drawings made of them, and which illustrate this paper. When Forbes was with us and produced his admirable work upon the anatomy of the Tubinares which were collected during the voyage of H.M.S. " Challenger " l, he found a great deal that was not only unique in the structure of Petrels, but in forms more or less nearly related to them. And I am of the opinion that when we come to examine carefully into the morphology of Arctic water-fowl, and more especially into that of their " soft parts," we shall discover much of interest, to say nothing of its importance as throwing light upon the organization of the types in question, as bearing upon the anatomy of the earlier forms of birds ; for it is among these groups, as we know, that we find many of the more lowly members of the class in point of structure and organization. This fact was never more forcibly brought to m y mind than after reading Forbes's investigations and observing tbe points I am now about to describe. In S. antiquus (fig. 1, p. 45) I find the lower larynx rather broad, and somewhat compressed from before backwards. The semirings of the bronchial tubes seem to be only partly formed in bone, while the last tracheal ring and the pessulus are completely ossified, the latter bar being V-shaped on the vertical section, with the apex above. What appears to me as most remarkable about this larynx is the mass of fat that overlies it in front,-and extends on to its posterior aspect, where it becomes thinner. This fat completely covers the tracheo-laterales muscles, which are inserted on either side into the middle points of the last tracheal ring. The sterno-tracheales are very large and lie embedded in this mass of fat. These are the only tracheal muscles present. Referring to B. marmoratus, fig. 2, we find the structure of the parts to be quite different. In the specimen before me, at least, there is an entire absence of fat from this part of the lower larynx. The anterior extremities of the lower tracheal ring, which is here, too, thoroughly ossified, do not meet so completely as they do in S. antiquus, or perhaps, more correctly speaking, this ring is roundly notched in front. B. marmoratus has a pessulus of a form corresponding very closely to the one described above for S. antiquus, but the tracheal tube above it is rather more cyclindrical, and not so much compressed from before backwards. The lateral tracheal muscles seem to agree quite closely in these two Auks, both as regards their size and points of insertion into the mid-lateral parts of the last tracheal ring, where they dilate slightly as they become inserted. Some considerable difference, however, is to be noted in the sternotracheal muscles of B. marmoratus, as will be seen in the figure ; they are given off much higher up on the trachea in this Auk, and are far slenderer than they are in S. antiquus. Unfortunately I neglected to examine the condition of the carotids in these two Auks before removing the viscera, as I was intent upon not injuring their very brittle skeletons, which had become much softened by soaking so long in the partially dissolved fat that encased 1 W. A. Forbes, Zool. Chall. Exp. vol. iv. pt. xi. p. 1. |