On a sacrum of a bird from the Wealden of Brook.

  • Seeley, H. G. (Harry Govier), 1839-1909.
Date:
1887
Catalogue details

Licence: Public Domain Mark

Credit: On a sacrum of a bird from the Wealden of Brook. Source: Wellcome Collection.

Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.

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    large A. Forsteri, which is not dissimilar in size, there is enough in common with the fossil to make some comparisons instructive. In the skeleton of Aptenoclytes Forsteri, in the Osteological Col- lection of the British Museum, numbered 1197 b, the presacral ver- tebra is imperfectly anchylosed to the sacrum, and comes away leaving a rugose surface, which is slightly convex from the veutral margin to the neural canal. This is the converse condition to that seen in the fossil. But just as birds have the vertebrae biconcave, flat, and opisthocoelous, as well as saddle-shaped in the interccntral articulation, there is no antecedent improbability in an extinct bird having the articulation procoelous. This Penguin’s sacrum contains ten vertebrae, and differs from the fossil in many details. It has not the median ventral canal, nor the pneumatic pits; but the Penguin is exceptional among water- birds in that respect. The transverse processes of the early sacral vertebrae of the Penguin are inclined forward as they ascend from the centrum to the neural platform, so that the process rises from one centrum to the neural arch in front of it; whereas in the fossil the processes are vertical, and given off at the junction of two centrums. The middle three sacral vertebrae of the Penguin, have the character already discussed as typical of existing birds, the attachments for the ilium being given off from the neural arch, and this is an important difference from the fossil. But tho posterior widening of the transverse processes seen in the Penguin is well marked in the fossil. And the last vortebra has the vertically ovate tubercle for tho ilium upon its own centrum, and is conditioned in every respect in the same way in the Penguin (fig. 6) and the fossil (fig. 5). The correspondence is almost as close in the character of the transverse processes and zygapophyses of the first sacral vertebra. The resem- blances further include the flattened neural platform, which is better developed in the fossil than in the Penguin. The neural spine in the fossil is thinner, more as in ordinary birds, and apparently lower than in the Penguin. Another resemblance is in the descent of the trans- verse processes from the neural platform in the first vertebra to the base of the centrum in the fourth vertebra, in consequence of which the first two vertebral have their bodies compressed from side to side (fig. 3). In both types the bodies of these vertebrae are of similar form, the first rather flattened on the underside and concave in length, the second widening as it extends posteriorly. Though the centrums in the fossil are approximately of equal length, their length diminishes in the middle of the sacrum in a way which is Avian, though the shortening is less than in existing birds. Prom the Avian form of the last sacral vertebra it may perhaps be legitimately inferred that the tail was not unlike that of a Penguin. Prom this comparison it is manifest that the differences which the fossil shows from existing birds are three : first, the small number of vertebrae in the sacrum ; secondly, the absence of the sacral recesses for the middle lobes of the kidneys; and, thirdly, tho form of the articular face of the first sacral vertebra. Possil birds lessen the importance of these differences: first the Archaeopteryx has as