134 ANATOMY OF THE WOOD RAT 



vertebra was not ankylosed with the third — the third and 

 fourth sternebrae are completely fused. In three Homo- 

 dontomys the length of the sternum is 42.6, 37.3, and 34.8 

 mm., or respectively 29.2, 27, and 26 per cent of the verte- 

 bral body length. In Neotoma these figures are 32, 31.5, and 

 34.8 mm. or 29.4, 30, and 33 per cent. In Teonoma it meas- 

 ures 46.8 and 43 mm., or 31.8 and 31.4 per cent of the body 

 length. 



The manubrium is centrally indented craniad, T-shaped, 

 with a body and two lateral processes upon the cranial end. 

 The body is longest in Homodontomys and much the shortest 

 in Neotoma, although there is considerable individual varia- 

 tion in this respect. The manubrial processes are, however, 

 largest in Teonoma. The first ribs articulate by facets in the 

 angle between the processes and body, and the second, in a 

 similar manner laterad upon the caudal end of the body. 

 The muscles most intimately connected with the manu- 

 brium are the sternofasciahs in Homodontomys only, and the 

 rectus abdominis and pectoralis superficialis in all three 

 subgenera. If there are any muscular stimuli that have 

 directly affected the subgeneric difference in the manubrium 

 they are obscure. 



The first sternebra is a trifle shorter than the manubrium, 

 and each of the others is shghtly shorter than the one next 

 craniad. They are a trifle the longest in Teonoma, and per- 

 haps shortest in Neotoma, while the posterior ones are heavi- 

 est in the former subgenus. The facets upon the caudal por- 

 tion of each are attached respectively to the third, fourth, 

 fifth, and sixth ribs, and the seventh are also attached to the 

 ventral portion of the fourth sternebra medio-caudad of the 

 sixth costal cartilages. The muscles arising upon this portion 

 of the midventral fine doubtless have some effect upon the 

 sternebrae, but the stimuli affecting them are regarded as 

 being more mechanical than myological, through the in- 

 fluences of the ribs. 



