ON THE SO-CALLED TWO-HEADED RIBS IN WHALES

AND IN MAN. BY PROFESSOR TURNER.

ALL anatomists, who have given much attention to the osteology of the Cetacea, are aware that in several skeletons of these animals which have been described, more especially in the skeletons of some of the whalebone whales, a peculiar bifurcated subdivision of the vertebral end of the first rib has been observed.

This anatomical peculiarity has been regarded by some systematic zoologists, more especially by Dr J. E. Gray of the British Museum, as a character of so much importance that it has been made a basis for classification. Dr Gray has separated those skeletons of the whalebone whales in which this condition of the first rib has been seen from the species with which they might in other respects have been associated, and erected them, not merely into distinct species, but even into new genera.

This position of Dr Gray's has recently been called in question by Prof. Van Beneden of Louvain, who argues that the bifidity of the first rib in a whale is, like the occasional occurrence of a cervical rib in man, “not a normal arrangement but an accidental conformation, of which zoologists need take no more heed in classification than of anomalies or monstrosities1" and that the genera Sibbaldius and Hunterius ought to be suppressed.

I propose in this communication to bring together and review those facts and opinions which have been collected and advanced by previous writers, and to record some new facts and observations, which may, I hope, prove of service in enabling naturalists to come to a satisfactory conclusion on this subject.

a. In Whales.-In the year 1864 Dr Gray communicated to the Zoological Society of London' an important memoir on the classification of the Cetacea, in which he subdivided the family Balaenopteridæ into five genera. To one of these he

1 Bulletins de l'Academie Roy. des Sciences de Belgique, 11. 1870, p. 320. 2 Proceedings, May 24, 1864.

applied the name Sibbaldus or Sibbaldius, and gave as one of its most diagnostic characters "the front ribs double-headed.” In this memoir he recognised two species in that genus. To one, Sibbaldius laticeps, he referred the skeleton of the whale in the Berlin Museum described by Rudolphi, and a skeleton at Leyden to the other, S. borealis, he referred the Ostend whale, and a female stranded in 1840 on the coast of Dorset and described by Mr Yarrell as Balaenoptera Boops'. He has since erected laticeps into a distinct genus, to which he has given the name Rudolphius3.

3

In the same year Prof. Flower communicated to the Zoological Society a description of several skeletons of the cetacea which he had examined in the museums in Holland and Belgium, and on one of these in the Leyden Museum, the skeleton of a whale taken ou the coast of Java, he founded a third species, Sibbaldius schlegelii. Three other species have also recently been added by Mr Cope, viz. S. sulphureus, tectirostris, and tuberosus, from whales which frequent the North Atlantic or North Pacific Oceans".

As it is of importance for our enquiry that we should possess an exact conception of the conformation of these so-called two-headed ribs, I shall now state concisely the nature of this anatomical peculiarity; and for fuller descriptive details refer the reader to the memoir by Prof. Flower already quoted, and to several short papers by Prof. van Beneden in recent parts of the Bulletins de l'Academie Royale des Sciences de Belgique.

In the greater number of the skeletons, referred to the genus Sibbaldius, in which this conformation has been observed, the first rib was subdivided at its vertebral end by a deep cleft into two distinct processes, one in front of the other, so that they lay in two different transverse planes. The posterior subdivision articulated only with the transverse process of the first dorsal vertebra, whilst the anterior articulated with the transverse process of the seventh cervical vertebra.

An interesting and instructive variation from this arrangement has been described by both these anatomists in a skeleton

1 Ibid. Feb. 11, 1840.

2 Catalogue of Seals and Whales, p. 170, and Supplement, 1871, p. 54.

3 Proceedings, Nov. 8, 1864.

4 Proc. Nat. Hist. Soc. Philad. 1869, and Gray's Supplement, 1871, p. 55.

of laticeps, obtained by Eschricht from the North Cape, now in the Museum of Natural History in Brussels'. Applied to, and articulating with, the anterior surface of the first right rib was an elongated compressed moveable bone, curved like a rib, whilst the corresponding bone on the left side was united to the left first rib by ankylosis. These bones were without doubt examples of supernumerary cervical ribs; on the one side fused with the 1st thoracic, and on the other a distinct bone; although it is not stated, either by Flower or Van Beneden, that they articulated by the vertebral end with the transverse processes of the seventh cervical vertebra.

From some of the descriptions which have been given of the Ostend whale, and from Rudolphi's own account of the skeleton of laticeps in the Berlin Museum, it might at first sight appear as if in these animals the two so-called heads of the first rib had not articulated with the first dorsal and last cervical vertebræ, but with the first and second dorsal vertebræ. Dr Gray, for example, in his summary of the characters of the Ostend whale, founding without doubt on the following passage in Dubar's Ostéographie, "mais la première étant biceps, elle s'articule aux deux premières dorsales,” states, "the first rib doubleheaded, articulated to the first and second dorsal vertebræ." But if Dubar's description be carefully perused it will be seen, that he says elsewhere, p. 38, that the two-headed first rib articulates with the transverse processes of the seventh and eighth vertebræ. Now as the cetacea, like the mammalia generally, possess seven cervical vertebræ, it follows that the seventh vertebra should not be regarded, as was done by Dubar, as the first dorsal, but as the last cervical, so that in this, as in the former specimens, the bicipital end of the rib articulated with the seventh cervical and first dorsal vertebræ.

Again, in Rudolphi's3 description of the specimen of laticeps in the Berlin Museum, he speaks of thirteen ribs on each side,

1 Van Beneden has figured the anterior ribs of this skeleton in the Bulletins, 1868, p. 16, Plate I.

2 Catalogue of Seals and Wales, p. 176.

8 Abhand. der Akad. der Wissenschaften zu Berlin, 1822, p. 27. Rudolphi named this whale Balana rostrata, but Dr Gray's specific name, laticeps, is more generally adopted.

but of fourteen dorsal vertebræ, so that he makes the vertebra, to which the anterior subdivision of the bicipital first rib is articulated, to belong to the dorsal series. Eschricht, however, has shown' that the skeleton of this whale possesses only thirteen dorsal vertebræ; consequently, the vertebra which Rudolphi considered to be the first dorsal is more correctly to be regarded as the last cervical.

Reinhardt also in his essay on Steypireyor, recalls attention to the fact that Eschricht pointed out, more than twenty years ago, an indication of a bifurcation in the upper end of the 1st rib of a whale, which he, and more recently Prof. Flower, have referred to the Balienoptera musculus; and during the past year Van Beneden observed a bifid 1st rib in the skeleton of a Balaenoptera musculus captured in the Scheldt.

But the Balaenopteridæ are not the only whalebone whales in which Dr Gray has founded a genus on the double-headed form of the first rib. In the Museum at Leyden is the skeleton of a Southern Right Whale from the Cape of Good Hope, in which Schlegel many years ago pointed out that each first rib was subdivided at its vertebral end by a deep notch into two processes. This skeleton has usually been referred to the genus Balana, or Eubalaena, sp. australis. In a second memoir on the classification of the whalebone whales Dr Gray erects it into a separate genus, confers on it a new specific name, and terms it Hunterus, or Hunterius, Temminckii. Since the publication of that memoir Prof. Lilljeborg has given an account of two subfossil whales discovered in Sweden". One of these he refers to the genus Hunterius, and distinguishes it by the specific name Svedenborgii, but he states that both the first and second pairs of ribs were missing from his specimen, so that we have no information on the form of the first rib in this creature. The skeleton of a young whale from the Museum at Pampeluna,

1 In Lilljeborg's Essay on the Scandinavian Cetacea, translated for the Ray Society, p. 269.

2 Annals of Natural History, Vol. 11. 1868.

3 Bulletins, 1870, p. 320.

4 In the articulated skeleton the two heads of this rib are attached to the 1st and 2nd dorsal vertebræ, but Prof. Flower has shown that they ought to have been articulated with the 7th cervical and 1st dorsal.

5 Annals of Natural History, Nov. 1864.

6 Nova Acta regia Societatis Upsaliensis, 1858, p. 35.

which Mr Flower states to possess a bifid first rib, has been named by Dr Gray Hunterius biscayensis1.

In the month of October 1870, the Anatomical Museum of the University of Edinburgh acquired, through the liberality of Messrs. J. and J. Cunningham of Leith, a number of cetacean bones, which had been imported into Leith from the Cape of Good Hope. On examination I found them to belong to at least two distinct species-the humpbacked whale of the Cape, Megaptera Lalandii, and the Southern Right Whale, Balana australis. One of the bones, a first rib, exhibited in a very characteristic manner the forked condition of the vertebral extremity. It may be useful for purposes of comparison to describe it carefully. It was subdivided at its vertebral end by a cleft twelve inches deep into an anterior and posterior limb. The anterior, curved from the body of the rib upwards and inwards, it was somewhat flattened on its anterior and posterior surfaces, and had its inner and outer borders rounded: the inner border formed a continuous curve with the inner border of the body of the rib. The posterior limb had unfortunately been broken away near the base. The body of the rib was 33 inches long from the bottom of the cleft to the sternal end, which latter had an irregular broken outline, and in all probability was not bifid, as in the rib of H. Temminckii figured by Dr Gray2. The breadth of the body of the rib, five inches from the bottom of the cleft, was nine inches, and ten inches from the sternal end it was fourteen inches, which was the broadest part of the rib. The outer surface of the rib was somewhat convex. The inner surface, slightly concave, presented a broad vertical ridge which commenced at the bottom of the cleft and extended for rds of the distance towards the sternal end: on the supposition that this bone was formed by the fusion of two originally distinct ribs, it is probable that this ridge marked their line of union. The circumference of the base of the anterior limb was thirteen inches, that of the posterior about sixteen inches. The chord of the arc from the vertebral to the sternal end was 41 inches.

In the toothed whales, also, the anterior ribs have occasionally been observed to present a similar modification in form; 2 Catalogue, p. 99.

1

Supplement, 1871, p. 44.