202 COPE. [Vor. III. 
presence of a sagittal crest, the only instance of its presence in 
existing selenodont Artiodactyla. The forms of bunodont Artio- 
dactyles with the orthal mastication (Elotherium and Dicotyles) 
have the molar series posterior ; those with lateral movement of 
the lower jaw (Sud@, Phacochwride) have them rather more 
anterior. In man, also, the dental series retains its primitive 
posterior position, but for very different reasons. The large 
development of the cerebral hemispheres has caused the reten- 
tion of the primitive foetal flexure of the cranium, and the 
anterior part of the dental arcs has even retreated from the 
position it occupies in the apes. The non-retreat of the inferior 
part of the mandibular symphysis has caused the appearance of 
the chin, a feature so characteristic of man. 
The use of the canine teeth as organs of prehension in the 
Carnivora is directly followed by the abbreviation of the muzzle, 
as in the bull-dog, the hyzenas, cats, etc. 
The mechanical cause of these various positions may be inter- 
preted as follows: In vertical action of the lower jaw, the use 
is severe directly as we approach the power; z.e. the point of 
insertion of the temporal and masseter muscles. The posterior 
teeth will then acquire the greatest development, and will con- 
tinue to be erupted at the region of the greatest nutritive 
activity, z.e. posteriorly. In the case of the Rodentia there is 
another mechanical influence, viz.: the posterior horizontal pres- 
sure exerted by the inferior series on the superior. Why the 
Proboscidia should retain the posterior position of the superior 
molar teeth, while the pressure of the inferior molars is in the 
opposite direction, ze. from behind forwards, is not readily 
explicable at present. But it is evident that, owing to the 
extraordinarily vigorous use to which the superior incisors are 
subjected, as in the case of the canines of the Carnivora, that 
natural selection would preserve the short-muzzled forms in 
competition with long-muzzled ones. 
The modification of the skull which the Cetacea and Sirenia 
have undergone with the lapse of geologic time, is well known. 
In the earliest known representatives of the order Cetacea, the 
Zeuglodontide, the external nostrils occupy a point on the su- 
perior face of the muzzle approximately half way between the 
lines of the premaxillary and anterior frontal borders, and 
the nasal bones are elongate. In the Balaenidz the frontal is 
