172 



PROCEEDINGS OF THE NATIONAL MUSEUM. 



VOL. 42. 



Horns. — The horns of the adults vary conspicuously in robustness, 

 in outline of cross section, in height to which the extremities rise 

 above skull, and in plane occupied by the abruptly curved tip.* 

 Most of these variations can be appreciated from the photographs in 

 plate 15. Diameter of horn at base, 30.5 by 23.5 in No. 173819, 

 33 by 25.5 in No. 173822, these specimens representing the extremes. 

 The cross section is usually flattened on the outer and posterior 

 aspects, the region of juncture sufficiently abrupt to produce an 

 evident angle or low rib extending from base of horn to beyond 

 middle. In No. 173818 the flattening is very slight, while in No. 

 173820 it is practically absent, so that the section is broadly ovate in 

 outline, wider posteriorly than anteriorly. The differences in the 

 curves and general direction of the horns is sufficiently shown by the 

 photographs of the six skulls which present the extremes of variation.'' 

 Though the abruptness of the curve at tip is subject to little variation, 

 the plane in which the curved portion lies may be nearly horizontal, 

 as in No. 173821, or nearly vertical, as in No. 173818. All inter- 

 mediate positions occur, and in this respect there is often, as in No. 

 173825, an appreciable difference between the horns of the same pair. 



Table of cranial measurements ofGazella Isabella. 



EXPLANATION OF PLATE 15. 



Skulls of six specimens of Gazella Isabella (about x^). 



No. 1, Cat. No. 173822. 



No. 2, Cat. No. 173819. 



No. 3, Cat. No. 173823. 



No. 4, Cat. No. 173818. 



No. 5, Cat. No. 173825. 



No. 6, Cat. No. 173821. 

 Note the progressive elevation of horns from No. 1 to No. 5. Plane of terminal hoot 

 nearly vertical in No. 4, nearly horizontal in No. 6. 



1 The abraptnes3 of this curve, one of the main characters separating aazella isabella from O. dorcas, is 

 perhaps the least variable feature of the horns. 

 * All of the figured specimens are adult with the exception of Nos. 173S21 and 173825. 



