Bulletin of the Nuttall Ornithological Club

by Nuttall Ornithological Club

With respect to the skull of Cinclus, our space will not permit us to enter upon the engaging part of the subject as to the mode of formation of this part of the skeleton in the adult from the many segments found in the cranium of the chick, it being enough for us to say that the usual bones ossify, unite, and leave the ordinary ones free, as the pterygoids, the ossa quadrata, and the lower jaw. The superior mandible is drawn out into a sharp point, and the bony nostril on either side occupies considerable space, being long and elliptical in outline; as in all nearly related genera these apertures are not separated by a bony partition or septum, but below we detect a delicate vomer in the median plane.

The eye-cavities or orbits are well shut off from the nasal chambers beyond them by broad bony walls composed of the usual elements, and here each is of a quadrate figure, as seen in so many genera of birds. The upper and outer angles of these osseous partitions are rounded. The almost complete separation existing between the two cavities just referred to by no means exists between the orbits themselves, for here we find an extremely deficient septum, and a large aperture leading into the brain-case at the usual site of the exit of the nasal nerves, the openings for the optic nerves being circular and entire.

On the inferior aspect of the skull we find maxillo-palatines, of a more or less spongy composition, existing between maxillaries and the delicate palatines, which latter are slightly bent downwards from the horizontal plane. The pterygoids are very slender, and articulate in the usual manner with the quadrates and the palatines.

The external form of the brain-case is more or less globular, the supra-occipital prominence being well developed behind. Above in the median line a shallow furrow is carried forward as far as the fronto-maxillary suture.

There is but little of interest to note in the lower mandible, to illustrate the points we have in view.

From this slight sketch of this part of the skeleton we are prepared to look a little into how Cinclus compares with other forms of near kin. The writer, to illustrate his remarks, offers the student the four accompanying cuts of the superior aspects of the skulls of birds chosen with the view of showing the comparable points.

A is of Oreoscoptes montanus, B of Sialia mexicana, C of Cinclus itself, and D of Siurus nævius.

In the figures, the angle formed at l, l′, b, and b′ is due to the lachrymal bone on that side; viewed from above in such forms as Sialia, Turdus grayi, Oreoscoptes, Hylocichla unalascæ, and no doubt Merula and Mimus, less so in Harporhynchus, this projection is markedly angular; while in Siurus, the Wrens, and rather less so in Anthus, it is rounded, as shown in Siurus and also in Cinclus itself.

Of the forms we have examined, Siurus appears to be closer to the Dipper in this respect than any other genus, the Wrens (Salpinctes) next, and Anthus next. This also applies to the manner in which the median furrow at the summit of the cranium approaches the fronto-maxillary suture, also shown in C and D in the cuts, this feature in the opposed forms mentioned above occupying a position between the superior orbital margins.