1875.] ON ASIATIC SHEEP, 511 
distinct name that will not only render intelligible Mr. Severtzoff’s 
descriptions, but may also be used in future in the comparisons and 
descriptions of these parts. 
The horn of an adult typical Sheep is divided by three more or 
less distinct edges into three surfaces. Of these latter (speaking of 
the basal portion of the horn), one is anterior (fig. 1, @), one exterior 
(fig. 1, 6), and one interior (fig. 1,¢). The first-mentioned of these 
surfaces we propose naming the frontal surface, the second the 
orbital surface, and the third the nuckal surface. These terms 
a. Frontal surface. d. Fronto-nuchal edge. 
b. Orbital surface. e. Fronto-orbital edge. 
ec. Nuchal surface. f. Nuchal edge. 
appear to us preferable to the terms anterior, exterior, and interior, 
as, owing to the spiral twist of the horns, the relative position of the 
surfaces is reversed in their basal and terminal extremities. Of the 
two edges which border the frontal surface, one (fig. 1, 2) is interior, 
forming at its origin the nearest approach of the horns; this we 
shall refer to as the fronto-nuchal edge, the other, the exterior 
(fig. 1, e), as the fronto-orbital edge. The remaining edge (fig. 1, f) 
we shall term the nuchal edge. 
Further, the entire spiral of the horn may be divided into three 
curves :— 
1. The basal curve ascends. 
2. The median curve descends. 
3. The terminal curve varies in direction according to the length 
of the horn. 
The angles formed by the axes of these curves, both with each 
other and with the vertical axis of the skull, have been used by Mr. 
Severtzoff as characters for distinguishing the different species. 
These we shall refer to as the basal, median, and terminal axis. 
