OSTEOLOGY 125 



heavy, 12 to 14 mm. in width, 8 to 9 in height, and does not 

 vary appreciably in form. Sahent points of the atlas are 

 the two transverse processes, the spine of the ventral arch, 

 and the spine of the dorsal arch. The ventral spine serves 

 chiefly as a point of attachment for the longus colh. Other 

 muscles attached to the atlas are the atlanto scapularis, 

 rectus capitis anterior, longus atlantis, recti capitis pos- 

 terior minor and laterahs, and obliqui capitis superior and 

 inferior. 



The axis is characterized by a well developed odontoid 

 process extending craniad upon its ventral arch, transverse 

 processes smaller than those of any other cervical vertebra, 

 and a very high, broad (in a sagittal direction), dorsal spine. 

 The atlas varies from 8.3 to 8.9 mm. in breadth, and 9.2 to 

 10 mm. in height. The spine in Neotoma is a trifle narrower 

 and higher in proportion, and its cranial part is relatively 

 lower in Teonoma, possibly indicating a weaker develop- 

 ment in the latter animal of one of the capiti muscles. To 

 the axis are attached the biventer cervices, spinahs dorsi, 

 rectus capitis posterior major, obhquus capitis inferior, and 

 very possibly a few fibers of some of the adjoining muscles. 



The second to fifth, inclusive, cervical vertebrae are 

 characterized by the breadth and flatness of the ventral 

 aspect. The pre- and postzygapophyses are present in the 

 second to seventh cervicals and of the usual form. In these 

 the length of the so-called transverse processes gradually 

 increases progressively in caudal sequence. They are single 

 — not branched — and extending first ventrad and then 

 caudad from the sixth is a well-developed inferior lamella. 

 The longus colli muscle extends both ways from this process 

 and may be chiefly responsible for the variation which it 

 exhibits, but the latter is beheved to be phylogenetic as well. 

 It is a trifle the best developed in Homodontomys, and per- 

 haps least so in Neotoma. The dorsal spines of the fourth to 

 seventh cervicals are poorly defined and that of the third 



