206 ANATOMY OF THE WOOD RAT 



consumption of a relatively tough, fibrous diet, Homo- 

 dontomys of one more brittle, and Neotoma of a fare more 

 intermediate in character between the two. Accessory to 

 these facts is probably the larger pterygoideus internus of 

 Teonoma and Neotoma, possibly slightly the best developed 

 in the latter, as shown by the more capacious pterygoid fossae 

 and longer angular processes of the mandible. These 

 muscles are of use in transverse motions of the lower jaw. 

 Of similar application is the fact that the digastricus, used 

 to pull the jaw backwards during mastication by the 

 molars, is largest in Teonoma; but the significance of the 

 variation in the distinctness of the tendinous division of the 

 muscle into two bellies is not known. The fossae from which 

 arise the transversus mandibularis are, perhaps, best defined 

 in Teonoma and least so in Homodontomys. This muscle 

 acts to control the mobility of the symphysis menti. 



The cervical vertebral series, expressed in percentage of 

 the body length, is of approximately the same relative 

 length in Hoinodontomys and Teonoma, but slightly shorter 

 in Neotoma, for just what purpose can not now be stated. 

 The inferior lamella of the sixth cervical seems if anything 

 to be best developed in Homodontomys and least so in Neo- 

 toma; but the longus colli, which doubtless is the chief muscle 

 influencing this process, appears to be best developed in 

 Teonoma. Many of the muscles of this region exhibit 

 subgeneric variation. The sternomastoid, cleidomastoid, 

 scalenus, longus capitis, longus colli, splenius, complexus, 

 and posterior portion of the biventer cervicis are all larger 

 or longer in Teonoma, and most of them also in Neotoma, 

 than in Homodontomys. The posterior portion of the bi- 

 venter cervicis and the longus atlantis may be a shade the 

 least efficient in Teonoma, while the rectus capitis lateralis 

 was recorded as smaller in both Neotoma and Teonoma. 

 These are all muscles which operate to move the head and 

 neck, and so, on the whole, these movements should be 



