2O8 NATURAL HISTORY ESSAYS 



has been longitudinally and vertically bisected to 

 show its structure. The face is remarkable for the 

 enormous development of the zygomatic or cheek 

 arches, which are expanded into a bony shield 

 almost concealing the lower jaw. This shield is 

 convex externally, and beautifully sculptured on its 

 outer side in a delicate tracery ; the sculpturing 

 seems to show individual variation. The skull of 

 the paca is only one quarter less in width than in 

 length. 



The death of a paca in the menagerie of the 

 Jardin des Plantes, about 1805, enabled Geoffrey 

 St. Hilaire to demonstrate the existence of extraor- 

 dinary cavities in the faces of these animals. 

 Daubenton had already noted the great size of the 

 cheek bones and their position, descending much 

 lower than usual ; while Buffon had observed a 

 longitudinal fold near the lower part of the upper jaw, 

 forming, as it were, a second mouth. St. Hilaire 

 showed that the fold was the opening of a fairly 

 large sac, passing right inside the face behind the 

 zygomatic arch ; it was lined by a continuation of 

 the skin, which was afterwards reflected outwards. 

 The sac was thus quite external to the mouth, 

 and resembled the outer pockets of a gopher rat 

 rather than the cheek-pouches of a monkey. 

 These gophers, it may be observed, have large 

 receptacles lined with hair and opening at the 

 lower edges of the cheeks, supposed to be used as 



