OSTEOLOGY 133 



for if the skeleton has been well cleaned, many of them will 

 have been lost, while if poorly cleaned, the smaller ones can 

 not be made out with certainty. 



The muscles concerned with the caudal vertebrae are the 

 lateral and medial extensors, the external abductor and the 

 short flexor. As the last two of these extend but as far as the 

 fifth vertebrae, and are the only ones which do so, they are 

 evidently chiefly responsible for the development upon 

 these only of diapophyses of noteworthy length. Both 

 Homodontomys and Teonoma are more scansorial, and there- 

 fore both employ the tail as an equilibrator. The bushy 

 tail of Teonoma offers more resistance to the air, and there- 

 fore, the more terete tail of Homodontomys must be more 

 strongly waved about in order to secure the same degree of 

 equilibrium for its owner. The development of the first five 

 caudal vertebrae shows that abduction and flexion of the tail 

 is more efficient in the latter subgenus, as might be expected. 

 Also according with this line of reasoning is the fact that 

 Neotoma, which climbs less and has less need for an efficient 

 balancer, has tail bones and processes that are somewhat 

 the lightest. 



Attention should be called to the fact that in the genus 

 Neotoma there is no definite elevation of the spines of the 

 vertebrae at any point posterior to the second or third thora- 

 cic. Special length of the spines at any point is an indica- 

 tion of particular strength in that region and is a character 

 exhibited by certain mammals fitted for a particular mode of 

 living. In this respect, therefore, the genus is indicated as 

 being definitely generalized in development. 



THORAX 



Sternum 



The sternum consists of the manubrium, the body, com- 

 posed of four sternebrae, and the xiphoid process. In a 

 single Homodontomys — the same in which the fourth sacral 



