1890.] The Cetacea. 601 
Paleontology confirms the inference derivable from their anat- 
omy, that the phylogeny of the Cetacea has followed the order, 
Archezoceti, Odontoceti, Mysticeti. 
The mechanical causes which may have given origin to the 
modifications which measure this succession, may be suggested as 
follows: The shortening and obliteration of the neck is probably 
due to disuse, since the genera! mobility of the body in a watery 
medium renders much flexibility of the neck unnecessary, the 
entire body being readily turned about. It may have resulted, 
also, from the increase in the relative proportions of the head, 
which renders it extremely difficult to handle ; a function which is, 
in the modern Cetacea, quite aborted. The early and rapid reduc- 
tion, and in some lines, extinction of the dentition, is a result of dis- 
use consequent on the increasing percentage of soft or minute food 
used by the more modern types. So the loss of the rib-heads in 
the Mysticeti may be traced to disuse, since, as above remarked, 
they lack the strain caused by the weight of the thoracic and ab- 
dominal walls and the contained viscera, which they experience in 
animals which are not supported by some external medium. The 
same reduction took place im the ocean-dwelling Plesiosauria,' 
and in those terrestrial reptiles in which the weight of the body 
is borne on the earth, as the lizards proper, and snakes. As re- 
gards the gradual transfer posteriorly of the external nostrils, the 
following mechanical hypothesis has been suggested. They have 
been used as a discharge pipe for air and water from the lungs 
and mouth, and, of course, facility of exit is directly as the short- 
ness of the conduit. It is possible that the constantly recurrent 
presence of a column of air and water on the posterior inferior 
wall of the nareal canal has literally pressed back this obstructive 
roof, until it has ceased to resist the outflow by becoming vertical. 
I. ARCHÆOCETI. 
This suborder embraces but one known family, which is de- 
fined as follows : : 
Frontal bones with flat, expanded supraorbital re- 
gion; teeth two-rooted posteriorly, one- 
rooted anteriorly ; Zeuglodontide. 
1 It must be remarked I that th y marine en have two-headed 
ribs, but chet of equal length, close together, e. 
