Bulletin of the Nuttall Ornithological Club

by Nuttall Ornithological Club

There is still another very marked distinction among the birds we have thus far compared, and that is in the general external form of the brain-case proper. A and B show the form assumed by the genera we mentioned above in connection with them; smooth, large, and globular, all indicating the possession of a brain of no mean size as compared with the owner. In Cinclus, Siurus, and the Troglodytinæ the prominence of the supra-occipital eminence causes depressions to exist at d and d′ that are not present in A and B at c and c′.

With regard to this last characteristic the outline assumed by Siurus seems to claim the nearer place, over the other forms mentioned.

So much for the skull, and the writer must reluctantly and with as good grace as possible allow the student to observe other interesting points of difference for himself, though he would be only too glad to assist him in this part of the skeleton.

There are fourteen cervical vertebræ in Cinclus, the last two bearing each a pair of free ribs, the ultimate pair possessing uncinate processes; this arrangement holds good in Siurus and Salpinctes, but we remember that in Eremophila[^[100]] we found only thirteen cervical vertebræ; the number of ribs varied however. Cinclus also possesses, in common with the form mentioned, four dorsal ribs; these are connected with the sternum by sternal ribs, the first sacral vertebra possessing an additional pair, but its sternal ribs only articulate along the hind border, on either side of the true sternal and last pair. This condition obtains, we know, in very many birds.

If we do not include the pygostyle or last coccygeal vertebræ, we observe that Cinclus has seven caudal vertebræ, Siurus and Salpinctes each only five, Oreoscoptes having six, so that the number of these segments may vary more or less among the genera we have quoted above.

The general pattern of the pelvis of the Dipper, the Wrens, the Thrushes, and Sialia is pretty much the same for all, that is it would be very hard to point out decided differences among them upon casual examinations; of course they are proportionate in size to that of their respective owners, and we might, in extensive series of each, by exceedingly careful measurements, detect relative differences. These remarks cannot be applied to the genus Harporhynchus, as the pelvis there has a very striking form, best expressed by saying that it is more angular than the others cited, the processes are more pronounced and sharper. In Cinclus, as in other forms noted, the bone is broad across, with the distal extremities of the pubic bones and ischia flaring well outwards; the ilio-neural canals open; the sacral vertebræ very broad, with numerous foramina or openings existing among them.

What we have just said in regard to the pelvis applies with equal force to the shoulder girdle and sternum; indeed, this latter bone is singularly alike among the various genera that I have referred to; the shape it assumes is that described by Professor Owen in his Anatomy of Vertebrates, as the “Cantorial sternum,” it being the pattern allotted to the vast majority of the class Aves. In front we find the manubrium bifurcated, and supported upon a stout and produced base, directed upwards and outwards. The body behind is 1–notched, the lateral xiphoidal processes thus formed having dilated ends. The keel is deep, convex below, sharp and concave in front, forming an acute cardinal angle at the point of meeting. The costal processes are very lofty, broad and directed forwards, having the facets for the sternal ribs placed along their posterior borders, which meet on either side the xiphoidal borders at a very obtuse angle. The “merry-thought” of Cinclus is delicately formed, having expanded upper extremities and a median plate below.

Our subject has, in addition to the usual number of bones in the pectoral limb, quite a sizable sesamoid, to be found at the back of the elbow; this bonelet is likewise found in Oreoscoptes and may be a common character of other birds we have mentioned. The arm seems to be completely non-pneumatic, indeed I have failed to find the apertures for the entrance of air in any of the bones composing it. Several months ago my attention was directed to a note, I think in the Proceedings of the Zoölogical Society of London, in which some English observer says the same of the European Dipper. This non-pneumatic condition of the long bones, not only of the upper but also of the lower extremities, seems to hold good among all the other forms and genera we have thus far referred to in this article.

The proximal extremity of the humerus is very much expanded, and rather abruptly bent in the direction of the bird’s body, the member being considered in a position of rest. The “crest” we know curls over the usual site of the pneumatic fossa, which depression is divided by a bony partition from a lesser cavity above. This characteristic is also more or less strongly marked in the Rock Wren, Siurus, and others, and is feebly shown in Harporhynchus.

The articular cavity of the shoulder joint is increased in the Dipper by a good sized os humero-scapalare, a sesamoid that we are aware is to be found among other orders.