572 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 



There are broad flange-like lateral plates or processes, perforate, as usual, for 

 arterial canals. The axis develops a stout, erect spine, overtopping that of 

 any other cervical excepting the 7th ; it is compressed, prominently and 

 sharply ridged anteriorly; ridged, but less sharply and prominently, behind; 

 and its apex is tuberculate. The cent ruin is small and much flattened; the 

 odontoid is well marked, and all the front of the body of the bone, including 

 the interior aspect of the odontoid, presents a continuous articular surface for 

 the atlas. The articular facets for the 3d cervical scarcely represent processes, 

 being simply borne upon the bases of the neural lamina;. The delicate 

 "transverse" processes are largely fenestrate with the circular vertebrarterial 

 foramina. The sixth cervical is peculiar in the points mentioned above. The 

 seventh cervical, as in human anatomy, is a "vertebra prominens", its spine 

 being abruptly longer than that of the preceding bone; it is more than half 

 as long as that of the first dorsal, which, in general appearance, it resembles 

 closely. In other points, this last cervical foreshadows the dorsal series. Its 

 transverse process stands straight out from the axis of the column, like that 

 of the 6th cervical, instead of obliquely backward, as in the rest of the cer- 

 vical series, and is notably longer than any antecedent one. The centrum is 

 abruptly narrower than the body of the 6th cervical, beginning that com- 

 pression and cylindricity which marks the dorsal and lumbar series. Further- 

 more, and chiefly, this last cervical vertebra is "dorsal" in character, in (a) 

 possessing no vertebrarterial canal, and (b) in bearing on the posterior border 

 of its centrum a demi-facet which takes equal share with that of the 1st dorsal 

 in the articulation of the 1st rib. 



Of the thirteen dorsals, the 1st is mainly discriminated from the last 

 cervical by the presence on the apex of the transverse process of a cupped 

 facet for the articulation of the tubercle of the 1st rib ; for we have just seen 

 how closely the last cervical simulates characters of a dorsal, even to taking 

 its share in bearing a rib. Its spine is, however, abruptly still longer ; its 

 transverse process is altogether stouter (besides bearing a facet) ; and its 

 body is narrower, longer, and more nearly cylindrical. The last (13th) dorsal 

 is distinguished from the 1st lumbar by presence of the facet for the last rib, 

 and by total lack of a small anterior prolongation or point of the "transverse" 

 process, which is readily recognizable upon the anterior lumbar, and becomes 

 very conspicuous on succeeding bones of that series. The centra of the dor- 

 sals grow longer, narrower, and more protuberant interiorly from the 1st to 



