GLIRES OF THE SANTA CRUZ BEDS. 463 



as lakes. In the mandible, dp T is more complex than the molars (PI. 

 LXIX, fig. 17); in addition to the principal internal valley there is a 

 second one, anterior to the former and of less vertical extent ; by the time 

 that the tooth is ready to be shed, this anterior internal valley is a mere 

 notch in the inner wall of the crown and may have disappeared altogether. 

 The three shallow external folds correspond in shape and position to those 

 of the molars, but have been worn away before the tooth is replaced. 



Skull. So far as I am aware, no entirely complete skull of this genus 

 has yet been found, but, by combining the various specimens, it is possible to 

 ascertain almost all the significant facts of structure. The skull has re- 

 semblances to those of both Dolichotis and Cama; it is narrower and more 

 slender than the former, especially in the cranial and interorbital regions 

 and the upper contour is straighten (The strongly arched appearance 

 seen in the specimen figured in PI. LXVIII, fig. 23, is chiefly due to 

 crushing, which has much depressed the parietal region. 



To a certain extent, the occiput is intermediate in character between 

 that of Dolichotis and that of Cavia, being lower and wider than in the 

 former, higher and narrower than in the latter; the foramen magnum is 

 relatively smaller and the median vertical crest less prominent than in 

 either of the recent genera, while the paroccipital processes are shorter. 

 As in Dolichotis, the periotic does not appear upon the occipital surface, 

 and apparently there is no mastoid process. None of the specimens dis- 

 plays clearly the limits of the periotic, but in the nearly allied genus 

 Schistomys this bone has much the same shape and connections as in 

 Dolichotis. The tympanic bulla is of only moderate size, and is relatively 

 less inflated than in the last-named genus, but with quite similar short 

 and incomplete tubular meatus. The parietal region is more or less 

 injured in all the specimens : so far as they are preserved, the parietals 

 have a smoothly rounded surface, without any trace of sagittal crest or 

 area, or temporal ridges. The supraoccipital is extended farther upon the 

 dorsal surface of the cranium than in Cama. The body of the squamosal 

 is larger than in the recent genera, but the supramastoid process is nar- 

 rower and there is no such groove above the root of the zygomatic process 

 as is seen in Dolichotis. 



The zygomatic arch is more elongate and the jugal is longer and more 

 slender than in the recent genera, as in which it does not extend to the 

 lachrymal ; in shape it has more resemblance to that of Cavia. The 



