1886.] Zoology. 295 
It follows, therefore, that soaring birds utilize zwo forces, viz., 
gravity and the motion of the atmosphere, and that without atmos- 
pheric motion other than that produced by his descent, a soaring 
bird cannot for an instant maintain, undiminished, his energy of 
position —/. E. Hendricks, Des Moines, Iowa, Jan. 14, 1886. 
On THE Types oF TooTH-sTRUCTURE IN MAMMALIA.—The trifid 
form of the lower molar of the genus Monachus and of Meso- 
nyx is in reality repeated in the cusps of the molars of Ungu- 
lata generally. In Sus the second upper molar cusp' sends a long 
basal process forward and outward toward the canine cusp and 
may be said to represent the anterior cuspule of the trifid figure 
of the plan of cusp itself, while the posterior cuspule is aborted. 
The bicuspid cusp exhibits a small anterior cuspule which ex- 
tends directly outward to reach the base of the canine cusp. Both 
the ridges formed by these cuspules are the first to become worn 
in mastication. This plan of arrangement is repeated in several 
extinct genera. among which may be mentioned Phenacodus and 
Hyracotherium. 
In Hyracotherium cuspidatum? the cuspule is placed obliquely 
to the true molar cusp, precisely as in Gymnura, instead of being 
continuous with the cusp. The differences between the cusp and 
the oblique prolongation of the first molar cusp forward and in- 
trifid figure. As may be easily supposed the direction of the 
ridges is subject to much variation. In man such direction is 
along a line which connects the first molar and the bicuspid 
The nomenclature of eS iad 
617) VAR Riv the cusps proposed by me in 1874 (Dental Cosmos, XVI, 
-ar Pe- Wheelers Survey, rv, p. 267, pl. Lxv, fig. 18. 
* Ibid, pl. LXV, fig. 1. oe EEF S : 
m vant Mammalia of Dakota and Nebraska, p. 245, pl. xxvit, f. 13, 15- 
61 ontography, P- 453. ] ` a 
Journ. of Acad. of Nat. Sci., 1874. 
