1889] NATURAL SCIENCES OF PHILADELPHIA. 19 



at the lateral aspect developing at this stage the elementary alveolar 

 " process " for the lodgment of the molar teeth. These latter are 

 now in a very primitive condition, though their form can be made 

 out with no little distinctness. As in the case of the incisor teeth, 

 their pristine sockets are larger than is necessary to lodge their 

 dental occupants. 



According to Wiedersheim, the jugal (Plate I, Figs. 1, 2 and 3 J) 

 is considered to be one of the investing bones of the outer side of the 

 mouth cavity, as the vomer is considered an investing bone of the 

 mouth cavity proper. In Xeotoma, as in some other rodents, the jugal 

 has an uncommon disposition, as it is overlapped by the zygomatic 

 process of the squamosal, and underlapped by the maxillary. For 

 the most part it is to be found lying along on the superior aspect of 

 the maxillary, while but a limited portion of it is to be seen upon a 

 better view of the arch, at the middle of its continuity. In neither 

 the embryo N. fuscipes, nor in adult specimens of the genus, does 

 the jugal possess any salient characteristics, its sole function being to 

 complete the slender zygomatic arch, and were it not that the de- 

 mands of a universal law exacted its presence, its actual use might 

 easily be dispensed with, for the zygomatic arch could just as well be 

 completed by a meeting of the maxillary and zygomatic process. 

 Indeed in some adult skulls of JY. cinerea, it takes a good lens and 

 careful observation to detect the presence of the jugal bone at all, so 

 perfectly is it moulded to the conformation of this delicate osseous 

 rod, which curves below and well out beyond the orbital cavity, in 

 one way defining its limits, and lending to the skull of this species 

 its well-known and characteristic form. 



The vomer presents nothing peculiar, contributing as it does to the 

 hinder moiety of the nasal septum, and ossifying rather late in the 

 growth of the skull. We may refer to this bone further on, when we 

 come to investigate the formation of the rhinal chambers. 



We next pass to a brief consideration of some of the most interest- 

 ing structures in this or any other mammalian skull ; I refer to the 

 mandible and those parts at its posterior extremity which are con- 

 cerned in the elaboration of the auditory apparatus. Here, to some 

 extent, we are upon debatable premises, and are brought face to face 

 with yet mooted questions in morphology, and consequently must 

 proceed with caution. Plate I, Fig. 1 shows very well the form of 

 the mandible upon its lateral aspect. At this stage it is deep and 

 thick, although the symphysis anteriorly has not as yet coossified, the 



