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The Morphology of the Sternum

1891; Nature Portfolio; Volume: 43; Issue: 1108 Linguagem: Inglês

10.1038/043269b0

1476-4687

G. B. Howes,

Paleontology and Evolutionary Biology

MY friend Prof. T. J. Parker has in these pages (Dec. 11, 1890, p. 142) lately recorded the existence of a sternum in the shark Notidanus indicus. The anterior of the two cartilages which he figures has been already described by Haswell (Proc. Linn. Soc. N.S.W., vol. ix., part 1); and, in view of Parker's conclusions, it is interesting to note that he speaks of it (p. 23) as “temptingly like the presternal,” but that “the presence of such an element in the skeleton of any group nearer than the Amphibia seems to preclude this explanation.” That the Amphibian sternum is for the most part, if not wholly, a derivative of the shoulder-girdle, there can no longer be a question; and, although the researches of Goette leave us in doubt concerning the hypo (post-omo) sternum, they show that that can be no derivative of the costal apparatus. Working anatomists will realize in Parker's application of Albrecht's terminology the expression of a fundamental difference between the sternal skeleton of the Ichthyopsida and Amniota. The researches of Goette, Hoffmann, Ruge, and others, show the sternum of the higher Amniota to consist of a greater costal portion and of lesser ones, chief among the latter being the episternum or interclavicle. They suggest (especially if Hoffmann's assertion that the precoracoid or clavicular bar is, in Mammals, primarily continuous with the spine of the scapula)that the interclavicle may be, throughout, the vanishing vestige of the coracoidal sternum of the Ichthyopsida. The latter would appear, therefore, to have been replaced in time by the more familiar costal sternum, derivative of the hæmal arches (ribs); and, this being so, might we not boldly, and with advantage, go a step further than Parker has done, and distinguish between a coracoidal archisternum of the Ichthyopsida, and a hæmocoracoidal neosternum of the Amniota? If this be conceded, the characters referred to must be incorporated in our diagnoses of the two great types named.