MUSEUM OF COMPARATIVE ZOOLOGY. 245 



converted in some instances into a vacuity. Upon the latero-extemal 

 walls of the posterior uares 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 cither the animal 

 is of so peaceful a nature that consolidated bones such as the frontals 



