630 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxxn. 



the infraorbital foramen is similarly placed in both genera. In the 

 characters as enumerated both the Aplodontidae and Mj^lagaulidae 

 differ from the beavers and squirrels. 



The palate is ver}'^ narrow between the large premolars, widening 

 more rapidly forward than backward. The space between the pre- 

 molars and incisors is comparatively short, with the premaxillaries 

 broadly rounded. The incisive foramen is placed well forward, as in 

 Aplodontia. 



The sj^mphysis of the lower jaw (Plate LVIII, fig. h) is small and 

 short. The jaw is heavy and deep in the region of the grinding teeth. 

 The angle, as shown in Plate LXIII, fig. ^, is broadly expanded and 

 inflected, but to a less degree than in Aplodontia. The coronoid is 

 comparatively high and straight, and is placed well back, capping the 

 base of the long, curved incisor. The condyle is small with a long 

 neck and irregularly rounded head. 



The nertebrse,. — The vertebral formula is, cervicals 7, dorsals 13, 

 lumbars 6, sacrals 5, caudals ?. The vertebra^ (Plate LXV) are com- 

 paratively short and heavy throughout, and the spinal column is 

 greatly curved, as in the beaver. The atlas (Plate LXIII, fig. c) is 

 imusually high and bears a short but well-defined spine. The poste- 

 riorly placed tranverse processes are blunt nodules of bone. The axis 

 (Plate LXIII, fig. 6?), is short and carries a high, robust spine, which is 

 widest antero-posteriorly at the base, but narrows rapidly and becomes 

 widest transversely at the summit. The other cervicals in the type 

 are only represented by fragments, but enough remains to show that 

 the zygopophyses are broad and flat and considerably overlap each 

 other in the same vertebra, indicating a considerable degree of flexi- 

 bility of the neck. The first nine dorsal vertebrae have long, slender 

 spines, but from this point they pass abruptly to the shorter, erect, 

 heavy spines of the lumbar region. 



The sacrum (Plate LXI, fig. a) does not dift'er materiallv from that 

 of Myla(jaulv)<. but is somewhat shorter, its attachments with the ilia 

 are not extended so far forward and the vertebrae are even more com- 

 pletely fused. In both genera the first sacral is much the largest. 

 The manner of fusion of the sacrum with the pelvis is suggestive of 

 some of the modern Edentates, although it is nearh' paralleled in 

 Spalax and some species of the Geomyidas ( Geomys jlorldmius). The 

 first two sacrals are anchylosed solidly with the ilia, the third is free, 

 while the transverse processes of the posterior two vertebrse are com- 

 pletely fused with the anterior portion of the ischium, thus forming a 

 larg'^ supra-acetabular foramen on either side. 



The ribs, probably thirteen in number, are comparatively heavy 

 throughout, their length and curvature indicating a large and well- 

 rounded thorax. The first and ninth, or tenth, ribs are shown in 

 Plate LXIV, figs, h and c. 



