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Show 118 ON THE HORN-GROWTH IN THE ST. KILDA SHEEP. [June 16, so persistently retained its colour (dark brown and black) and characteristic four horns which exist in both sexes. The irregular-development and somewhat erratic growth of these horns have been noted and commented upon by both veterinary surgeons and comparative anatomists; and some diversity of opinion exists as to the homology of the two pairs of horns when compared with the single pairs of horns as carried by some Domestic and Wild Sheep. The purpose of this communication is, from a biological point of view, to trace the cause of the duplicate pair of horns and to determine their homology. Text-figure 9, B, represents a typical well-grown head. " A n examination of a number of heads in the British Museum and the Museum of the Royal College of Surgeons and elsewhere shows an extraordinary variation in the form and direction, or ' pitch' as it were, of the median or upper pair of horns. These may grow quite upright, as shown on the left side of E, or may be curved forward to a greater or- less degree, but are always present. " The lateral or lower pair of horns, though more constant as regards form and more nearly approaching the typical horns of the Domesticated Sheep, are more subject to arrest in development from a variety of causes and are often absent. The apparent cause of the reduplication in the horns of this breed, in a considerable number of instances, is the splitting or segmentation into two or more nodules of the centre of ossification of the frontal bone, as indicated by three examples exhibited and by the skull (text-fig. 9, E). It is upon this point or ' boss' that the horns of all Ruminants ultimately grow, its prolongation forming the ' pedicle' of the Cervine horns and the ' horn-bearer' or core of the Hollow-horned Ruminants. " The lateral horns, on the other hand, though bearing a closer resemblance in form and position to the typical horns of the Domestic Sheep, are variable and erratic in their development. Frequently only one is present (as in text-fig. 9, C), or they may be altogether absent, leaving only the median pair, which then assume a typical form (text-fig. 9, A). " In-breeding, as recorded by the history of three small herds of these sheep, causes a reduction or arrest in development and often absence of the lateral horns (as in text-figs. 9, C and D), the median or upper pair of horns remaining constant. " Castration has a precisely similar effect. The totality of evidence gathered from the life-history of the breed would seem to indicate that it is the median pair of horns, notwithstanding their variation and abnormal appearance, winch are homologous to those of the Domesticated or Wild Sheep." Examples of three or four horns in the Domestic and Wild Sheep or Goat as well as in Cattle and Deer are of sufficient interest to the anatomist to merit record. In Proc. Zool. Soc. 1879, p. 803, there is a good figure of a skull of the Chamois with four horns. In the Museum of the College of Surgeons there is a skull of a Goat with four horns. In the current volume of Proc. |