GLIRES OF THE SANTA CRUZ BEDS. 451 



groove, which in the upper teeth is internal and in the lower teeth exter- 

 nal ; the latter, except usually m^, have masticating surfaces of somewhat 

 pyriform shape, the postero-internal end being narrow and compressed. 

 The last upper molar differs from the others in having a third, posterior 

 lamina, the development of which differs in the various species and is also 

 subject to considerable individual variation. The two rows of upper teeth 

 converge anteriorly though not so strongly as in Perimys. In the lower 

 grinding teeth there is not only a well defined external vertical groove, 

 but the anterior lamina projects outward beyond the posterior one, espe- 

 cially in m ? , in which the posterior lamina is very narrow; these teeth 

 have no distinct internal side, the anterior side curving gradually into the 

 posterior without angulation. 



Though the skull is far from being completely known, it evidently 

 agrees in essentials with that of Perimys and Viscaccia and is rather more 

 like the latter than the former, while in some respects it is intermediate 

 between the two. The occiput is like that of Perimys in shape, in the 

 median convexity and in the absence of any vacuity above the foramen 

 magnum, but the bones are thinner and the periotic is more extensively 

 exposed upon the occipital surface, though less than in JSiscaccia, and the 

 mastoid process is very small. As in the latter genus, there is no fossa 

 between the squamosal and exoccipital, this region being made convex 

 by the protuberance of the periotic, along which runs the continuation of 

 the occipital crest, which is less prominent than in the modern genus. 

 The condyles have the same high and narrow shape as in the latter, but 

 the paroccipital processes, which are broken in all the specimens, were 

 evidently smaller. Like that of Perimys, the tympanic bulla is largely 

 inflated, and the opening into it, below the auditory meatus, is also large ; 

 anteriorly the bulla sends forward a process to connect with the pterygoid. 

 The supramastoid process of the squamosal is relatively wider than in 

 Viscaccia and the sagittal and temporal crests are more prominent, while 

 laterally the parietals are more concave. 



The rostrum differs much in its proportions in the various species ; the 

 nasals are quite flat anteriorly and do not show the foliate shape seen in 

 Perimys and Viscaccia. At least, this is the case in P. profluens, the only 

 species in which the nasals are known. The premaxillae are shorter and 

 the predental portion of the maxillaries longer than in the recent genus, 

 and the fossa for the preorbital part of the masseter muscle is shallower, 



