506 WILLIAM H. F. ADDISON 
system. This was the opinion of Tiedemann (’26) and also 
of Elliot Smith (02), but another explanation may be offered. 
The fact that, in the process of adjustment of the organism 
to the new environment, the external respiratory opening has 
come to open at the dorsal surface of the head in these animals, 
instead of at the extremity of the snout, may also be a factor 
in limiting the growth of the brain anteriorly. When one exam- 
ines the cranium, the air-chambers are seen to descend directly in 
front of the cranial cavity. In an animal which must come to the 
surface to breathe every few seconds, it is evidently convenient 
to have the air-openings on the dorsal surface of the head, and 
equally desirable that the pathway to the pharynx and larynx 
should be most direct, and this is certainly helped by the short- 
ness anteriorly of the brain-box. Thus there may be a corre- 
lation between the change in position and direction in the air- 
passages, and the peculiar shape of the brain, entirely apart 
from the presence or absence of olfactory structures in the brain. 
In the Mystacoceti (baleen whales) there is a diminutive ol- 
factory bulb and peduncle, but in some of the Odontoceti (toothed 
whales) these structures are wanting. This is the case in the 
adult dolphin, where there is not the slightest trace of olfactory 
nerves, even in microscopic sections. In Globiocephalus melas 
there is reported to be complete absence of external olfactory 
structures in the brain by Pettit (’05), and this is the condition, 
too, in Beluga (Delphinapterus) according to Kikenthal and 
Ziehen (’89), who state, also, that there is a slight development 
of the olfactory structures in the embryo. In Tursiops trunca- 
tus, specimens of which lived for some time in the New York 
Aquarium, no olfactory bulbs and tracts were seen when the 
brains were removed in the laboratory of Prof. G. S. Hunting- 
ton. Although Delphinus is destitute of olfactory nerves, others 
of the family Delphinidae possess them. Thus, in Phocaena 
(Edinger ’11) there is a microscopic trace of these structures, 
and in Hyperoodon of the family Platanistidae there are also 
small olfactory nerves (Kiikenthal and Ziehen, ’89). Thus 
among the toothed whales, some have small olfactory structures 
while others have none at all. 
