OSTEOLOGY 131 



form a solid unit through the fusion of their diapophyses. 

 and to some extent of the other processes as well, except in a 

 single specimen of Homodontomys in which the fourth re- 

 sembles the first caudal vertebra and is similarly articulated. 

 In Neotoma lepida stephensi, and to a lesser extent in the only 

 Neotoma pennsylvanica available, the diapophyses of the 

 sacrals are not completely fused with their neighbors. 



Anapophyses as such are lacking in the sacral series. The 

 diapophyses of the first are prolonged and very stout, 

 and with the cranial half of the diapophyses of the second, 

 articulate with the ilia. The metapophyses of the first sacral 

 vertebra are large, but those of the remainder, together with 

 their postzygapophyses, are much reduced in size and are 

 insignificant. The sacral series averages a trifle broader, 

 perhaps, in Teonoma, but the individual is greater than the 

 subgeneric variation in this respect. The shortest interilial 

 measurement at the point of articulation varies from 8.5 to 

 nearly 12 mm. The breadth across the diapophyses is from 

 2 to 3.5 mm, greater in the posterior than the anterior por- 

 tion of the series in old Homodontomys and Neotoma, while in 

 Teonoma the two measurements are approximately equal. 

 These points are doubtless influenced partly by variation in 

 the development of the multifidis spinae and caudal extension 

 of the longissimus dorsi, although they undoubtedly also 

 reflect certain pelvic differences. Other muscles definitely 

 concerned with the sacral vertebrae are the medial and 

 lateral extensors, and the long and short flexors, of the tail, 

 and to a much lesser extent, the pyriformis and biceps 

 femoris anticus. 



Caudal vertebrae 



The caudal vertebrae vary in number with the individual. 

 In the articulated skeletons at hand there are 31 caudals in 

 four specimens of Homodontomys and 28 in one; in Neotoma, 

 28 caudals in three individuals; in two Teonoma there are 26 



