600 | The American Naturalist. [July, 
The line of successional modification of the Cetacea is found in 
changes in (1) the shape of the skull; (2) the extinction of the 
dentition ; (3) the shortening of the cervical vertebrae ; and (4) in 
the separation of the ribs from articulation with the vertebral 
centra. The modification of the shape of the skull is related to 
the gradual transfer of the external nostrils to more and more 
posterior positions, until they remain, in the extreme types, above, 
or even behind above, the eyes. In this process the nasal, frontal 
and parietal bones become excessively abbreviated, so that in the 
modern toothed whales, they form a narrow band between the 
nostrils and the superior border of the occipital bone. 
The order is naturally divided into three sub-orders, which are 
defined as follows : 
External nostrils on the superior side of the muzzle; 
teeth present; ribs with two heads; Archeoceti. 
External nostrils above gullet; teeth generally pres- 
ent; no whalebone; some of the ribs with two 
heads ; Odontoceti. 
External nostrils above gullet; teeth wanting; the 
gums supporting “whalebone” ; ribs articulating 
by tubercle only ; Mysticeti. 
All of the above, characters are those of divergence from the 
principal mammalian stem, and have relation to the conditions of 
aquatic life. Thus the posterior position of the nostrils permits in- 
spiration without the elevation of the muzzle above the water-level, 
which is rendered difficult, if not impossible in the most specialized 
types, by reason of the extreme flatness and inflexibility of the cervi- 
cal vertebra. The absence of teeth is appropriate to the habits of 
the types which lack them. Thus the Physeteride among Odon- 
toceti feed principally on squids, whose soft bodies are swallowed 
whole. The Mysticeti feed on minute Crustacea and Mollusca, 
which they retain in the mouth by straining the water through 
their bristly whalebone, or baleen. The disarticulation and dis- 
appearance of the heads of the ribs in the Mysticeti, is appropriate 
to the support which all the viscera derive from the fluid medium 
in which these large animals live. Strong articulation of the head 
of the ribs to the vertebral column is no longer necessary. 
