MUSEUM OF COMPARATIVE ZOOLOGY. 245 
converted in some instances into a vacuity. Upon the latero-external 
walls of the posterior nares large oval notches or deficiencies occur. 
Primates. — The sphenoidal and sphenomaxillary fissures in man, as 
also the foramen lacerum medium, which is often very large and irregular 
in shape, come under the second category of vacuities. The same also 
applies to the anterior tympanic of the higher Apes. 
In studying the etiology of these cranial vacuities in the Mammalia, 
it would seem at first sight, especially if the imperfect osseous condition 
of the skull in many of the lower Vertebrates be taken into consideration, 
that their existence was due to phylogenetic degeneration, taken in 
the widest acceptation of the term. A closer study of the conditions 
presented, however, apparently limits this degeneration to one of en- 
vironment. Take as an illustration the Pinnipedia, in which group 
the economy of nature as regards the disposition of material is admira- 
bly shown. ‘These true Carnivora have become adapted to an exist- 
ence in water. Consequent changes suited to this aquatic life have 
been undergone. The thick skull of the Fissipedia is no longer neces- 
sary ; consequently the cranial walls have become much thinned, and 
several large and extraordinary vacuities in different regions, notably 
in the basal, have been formed. 
A reduction has taken place in the number and strength of the teeth, 
and other characteristics of the order have been materially altered or 
entirely lost. These changes have been foreshadowed in the Enhydra 
(Sea Otter), as has already been noted. 
So also any diminution in the strength of the cranial walls of the 
ancient branch of the Ungulata, the Sirenia, due to large deficiencies, 
is counterbalanced by the aquatic habits of the order. 
In the Rodents, the alveolar border of the maxilla is pushed far back, 
and thus the unoccupied space between the incisors and premolars is 
relatively large. This absence of teeth necessitates only a small supply 
of bony material in the immediate region, so that in the case of the 
Hare, which feeds upon a succulent diet, little strength or resistance is 
demanded, and consequently large vacuities are found in the latero-nasal 
region. The nasal plate of the maxilla is rendered lighter by the 
reticulation, which may be considered as a series of small vacuities, 
while it still serves as a protection to the delicate structures beneath. 
Again, the presence of the vacuity in the side-walls of the face in many 
of the Ruminants, and in consequence the apparently weakened condition 
of the parts, may be explained by the statement that either the animal 
is of so peaceful a nature that consolidated bones such as the frontals 
