118 ox THE HOEN-GROWTH IN THE ST. KILBA SHEEP. [Juiie 16, 



SO persistently retained its colour (dark brown and black) and 

 characteristic four horns which exist in both sexes. The irreg'ular 

 development and somewhat erratic growth of these horns have 

 been noted and commented upon by both veterinnry 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 piirpose 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. 



" An 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 degi-ee, 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 suljject 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 consider- 

 able 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 

 Ptuminants ultimately grow, its pi'olongation forming the ' pedicle ' 

 of the Cervine horns and the ' hoi'n-bearer ' or core of the Hollow- 

 horned Riuninants. 



" 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 eriutic 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 ai'rest 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 efiect. 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 a,nd abnormal appearance, which 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 sufiicient 

 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. 



