INOS 25] THE HARD PARTS OF THE MAMMALIA. 203 
crowded back to the parietal, and the latter forms but a narrow 
band in front of the occipital, while the nasals form but a short 
roof over the nostrils, which are now above the eyes. In the 
Delphinidz the nostrils have an even more posterior position, 
and the nasal bones take part in the extreme shortening already 
seen in the frontals and parietals. The convenience of this 
location of the nostrils is evident. It relieves the animal of the 
necessity of raising the muzzle from the surface of the water in 
taking an inspiration ; a convenience which becomes a necessity 
in the two families last named, when the cervical region has 
become so short as to render such a movement impossible. 
The mechanical energy which has caused this posterior retreat 
of the nostrils appears to me to have been the constant pressure 
of a column of water from below, z.e. from the mouth. This 
constant discharge through the nostrils appears to me compe- 
tent to force the oblique superior wall of the nasal canal to 
become a vertical one, and to shorten the nasal bones from 
before backwards, until they cease to obstruct the outgoing 
current. 
y. Lhe Horns. 
Horns are developed in Mammalia and other Vertebrata on 
similar parts of the skull, principally on the posterior lateral 
angles, as in various Batrachia, Reptilia, and Mammalia, and on 
the nose, as in a few Mammalia and several reptiles, recent and 
extinct. These parts are the ones which are especially brought 
into contact with resisting bodies; the nose in pushing a path 
or way for the head and body; the lateral occipital region in 
defence and assault, when the sensitive nose and eyes are pro- 
tected by being held near the ground. In the latter position 
the postero-lateral angles, when present, receive more frequent 
collision with, and vigorous stimulation from, a body attacked or 
resisted, and in accordance with the observed results of irrita- 
tion on dermal and osseous tissues, additional matter has been 
deposited. In Lacertilia and Batrachia Salientia there are dis- 
tinct postero-external cranial angles ; in Batrachia Urodela such 
angles are less prominent. In Unguiculate Mammalia and in 
all others with a sagittal crest there are no such angles; hence 
this type of skull has never developed posterior horns. The 
rhinoceros has developed the dermal nasal horn, and the Elas- 
