87 



not to the extraordinary extent witnessed in Saccomyidce, where the 

 whole bone is blown up like a bladder. The squamosal roofs over most 

 of the cranial cavity ; and alone forms (with the exception of a little 

 place occupied by the interparietal) the whole occipital or lambdoidal 

 crest. The mastoid, which persists distinct from both squamosal and 

 occipital, though usually fusing with the petrosal, is immensely de- 

 veloped, its superficies lying mostly in, and representing about half of 

 each side of, the occipital surface. It develops a moderate " mastoid 

 process," lying against the postero external corner of the squamosal, 

 and looking like a duplicate of the paroccipital process that lies against 

 its opposite extremity. The petrosal does not share this unusual devel- 

 opment, the bulla ossea being, in fact, smaller than they are in Arvicola, 

 for instance ; they swell bat little below the baso-occipital plane. The 

 tympanic developes into a tubular meatus, set quite free from its sur- 

 roundings in a deep recess of the squamosal. The petrosal likewise is 

 fissured away from the squamosal, but in adult life the tympanic, pet- 

 rosal, and mastoid are consolidated. 



The upper and lower parts of the occipital bone are at riglit angles 

 with each other; the basi-occipital is horizontal upon the floor of the skull, 

 w^hile the superior and lateral elements are perpendicular behind. The 

 supra-occipital is squarish,with rounded corners; the ex-occipitals develop 

 into moderate obtuse processes. ^S^early all of the foramen magnum is 

 vertical ; the condyles are rather small and widely divergent superiorly. 



The suture with the basi-occipital, which persists for some time, is 

 ordinarily the most conspicuous of the sphenoidal relations which may 

 be appreciated in examination of adult skulls. Close inspection, how- 

 ever, shows the squamo-sphenoid suture just inside the glenoid fossa; 

 the alisphenoid barely misses taking a part in the mandibular articula- 

 tion (as in some marsupials) ; the orbito-sphenoid, lining the orbit be- 

 hind, rises nearly to the top of the skull. 



Tiie mandible remains for consideration. This is eminently charac- 

 terized by its massiveness and the emphasis of its various ridges and 

 angles. Nevertheless the symphysis, though extensive, is incomplete. 

 Instead of an edge below, the bone presents a broad, smooth, flattened 

 area, bounded on the sides by a ridge indicating the limit of masseteric 

 muscular attachment. The angle of the jaw is strongly exflected in a 

 lieculiar way. An oblique plate (the "descending process" in many ro- 

 dents) aiises from the inner side of the body of the bone, and curves 

 strongly backward and outward, ending far exterior to the main part of 

 the bone as a strong laminar process. Just inside of this, between it 

 and the condyle, there is a strongly marked, smooth, upright protuber- 

 ance. This is where the root of the incisor pushes up from the inside. 

 To the inner side of this knob, again, rises a third protuberance ; it is 

 the condyle, rather small and of no noteworthy features. (It appears 

 particularly small when compared with the glenoid cavity, which, as [ 

 should have remarked before, is of unusual width.) Thus the mandible, 

 viewed from behind, j>resents the curious appearance of three prongs — 

 condyle, incisor knob, and exterior process. The appearance of trifurca- 

 tiou is best marked in Thomomys, where the tooth-knob is most promi- 

 nent, and separated by deepest notches from the processes between which 

 it stands. In addition to all these prominences, a slender falcate acute 

 coronoid rises in front, and overtops the rest, being separated from the 

 condylar ramus by a deep notch. There is a deep excavation between 

 the thin laminar basis of the coronoid and the molar alveolus. The fora- 

 men of the inferior maxillary nerve appears on the inner side of the 

 root of the condylar ramus. 



The dental formula has been already given. The molar dentition 



