368 ANATOMY OF VERTEBRATES. 



outward in close connection with the descending process of the 

 large subquadrate mastoid, 8, which anchyloses with the petrosal 

 and tympanic. The long bony ^ meatus auditorius ' ascends 

 obliquely backward — the direction in which this timid Rodent is 

 most concerned in ascertaining the sounds that may warn it of an 

 approaching enemy. The tympanic cavities intercommunicate by 

 a sinus traversing the basisphenoid. The outer part of the ali- 

 sphenoid is j^erforated by the ectocarotid artery. The entocarotid 

 pierces the tympanic bulla. The petromastoid is articulated in a 

 peculiar manner to the squamosal, which, after expanding beyond 

 its zygomatic part to be applied to the parietal and alisphenoid, 

 resumes the form of a narrow thin plate of bone, applied to a 

 shallow depression upon the mastoid, and thus clamping it, as it 

 were, to its place. The frontal sends outward a large aliform 

 curved plate above each orbit, the extremities of which form 

 postorbital and antorbital processes, the notches which divide the 

 anterior from the posterior part of the frontal being unusually 

 deep. The common outlet of the optic nerves extends forward, 

 so as to occasion a small vacuity at the back part of the inter- 

 orbital septum. Each orbit presents a wide vacuity at its fore 

 part, which leads into the lateral nasal cavity, bounded externally 

 by the singularly reticulate nasal plate of the maxillary, 21. The 

 zygomatic arch, which is slightly curved downward but scarcely 

 at all outward, developes a small prominence both from its front 

 and hind extremity below the points of suspension. The arti- 

 cular surface for the lower jaw is broad and concave transversely, 

 narrow and convex longitudinally. The bases of the sockets of 

 the superior molars form a strong prominence in the orbit below 

 the anterior vacuity. The nasal bones, 15, are remarkable both 

 for their length and breadth : they extend further back than the 

 long slender nasal processes of the premaxillaries, 22. The bony 

 palate is extensively encroached upon by the prepalatal apertures, 

 which blend together to form a narrow heart-shaped vacuity 

 with the apex directed forward, largely exposing the vomer 

 and the nasal cavities. The palatal processes of the maxillaries 

 and palatines form a bridge, or platform, extending across oppo- 

 site the three anterior molar teeth. The nasal processes of the 

 palatines are of unusual height. The angle of the lower jaw 

 forms a broad compressed plate, with the lower border rounded 

 and thickened, so as to project a little beyond both the outer and 

 inner surface of the ascending plate : the outer ridge is continued 

 forward to the horizontal ramus, bounding the large masseteric 

 fossa. The petrotympanics form ' bullae osseas.' The pterygoids 



