257 



by the occipital. The extent of this narrowly elliptical transverse inter- 

 parietal is greater than in Mus; its corner is at a point where the back 

 onter angle of the jiarietal, back upper angle of the squamosal, and 

 front upper angle of the occipital all come nearly together. The squamo- 

 sal closely resembles that of 3[us in size, shape, and connections ; there 

 are the same extensive vacuities about the petrosal, with a similar strong: 

 clasp, bridging over the opening just above the meatus, running from 

 the root of the zygomatic process to tbe back edge of the bone. The 

 mastoid is of moderate size, developing nothing to be fairly called a 

 process, wedged between the paroccipital process and the squamosal, 

 at the postero-lateral corner of the skull. It is confluent with the petro- 

 sal, but partially fissured away from the surrounding occipital elements. 

 The supraoccipital is of large size and convex contour ; the occipital 

 crest is slight, so that the plane of the occiput is not well defined from 

 that of the superior surface of the skull, the two meeting with a contin- 

 uous curve, more convex than in 2£us. The upper border of the occipital 

 is nearly straight, and bounded quite across by the interparietal ; next 

 comes a considerable piece of squamosal suture, and then the mastoid. 

 The foramen is of great size and nearly hexagonal shape ; most of it 

 being in the one plane of the occiput, with only a slight nick inferiorly. 

 The condyles are protuberant and convergent ; the condyloid foramen is- 

 close beneath their articular surfaces. The paroccipital are well-marked 

 vertical processes. The basioccipital narrows very rapidly, owing to 

 the strong inward trend of the i^etrosals, and ends by transverse suture, 

 as usual, with the basisphenoid, opposite the ends of these bones. Its 

 under surface shows a pair of slight depressions, with a median ridge. 



The posterior nares are of ample dimensions, owing to the wide sep- 

 aration of the pterygoids. These bones are long, straight, and styloid,, 

 with a slightly-clubbed extremity in close approximation to the ends of 

 the petrosals. The palate ends behind with a broad, rounded emargina- 

 tion opposite the last molars. This formation is very different from that 

 of Mus, in which the bony palate extends back of the molar series, and 

 the contracted inter pterygoid space is narrowly angular. The maxillo- 

 palatine suture of Zcqms, likewise, is differently located, being opposite 

 the interspace between the penultimate and preceding molar, instead of 

 much farther back. There is a pair of conspicuous palatal foramina op- 

 posite the penultimate molar. The contour of the palate differs from 

 that of 3fus, and perhaps a majority of allied rodents, in being broader 

 in front than behind. The incisive foramina are of great length, as well 

 as quite broad, reaching from little behind the incisors to opposite the 

 molars; the perforation is half in the intermaxillary, half in the max- 

 illary ; the bony septum is bullous except at its posterior part. 



The form of the descending process of the mandible is a strong char- 

 acter of Zapus in comparison with Mus, &c., in which this plate of bone 

 is more or less squarish, and vertical or nearly so. In Zapus, the same 

 plate is strongly twisted out of the axis of the jaw, standing diagonally 

 outward and upward — very much, in fact, as witnessed in the Saccomyidce. 

 The coronoid is rather weak, falcate, acute, with a strong slope; it slightly 

 overtops the condyle. The latter sets strongly backward, though it 

 is rather more erect than in Mus. The incisor causes a moderate protu- 

 berance outside, at the root of the condylar process. Inside, nearly 

 opposite, is the conspicuous foramen of the inferior maxillary nerve. 



The superior incisors are short and stout, with a strong curve ; their 

 anterior faces strongly sulcate, with the outer half of the tooth rab- 

 beted down so that the groove is plainly visible from the side. The infe- 

 rior incisors are not specially noteworthy. The molar series differs from 



