SKELETON OF RODENTIA. 377 



muscles seem to have been unusually small in this Rodent : their 

 fossae impress only the small squamosals. The coronoid process 

 of the lower jaw is obsolete. The movements of the jaws appear 

 to have been chiefly committed to the masseteric and pterygoid 

 muscles. The zygomatic arch, which extends from the squamosal 

 to the premaxillary, is very broad below the orbit, and is traversed 

 externally by a ridge indicating the powerful origin of the mas- 

 seter. The antorbital vacuity and the maxillary depression, 

 bounded externally by the two roots of the zygoma, are larger 

 than the orbits : the front root of the zygoma is formed by a 

 combination of the frontal, lacr3^mal, maxillary, and malar bones. 

 The slender extremities of the premaxillaries terminate on nearly 

 the same transverse line vnih the back part of the broad nasals. 

 These are bent down anteriorly, so as to form the sides of the 

 external nostrils. The deep sockets of the rootless teeth form 

 protuberances at their bases, where the osseous case becomes 

 absorbed, converting the socket into a canal open at both ends, 

 the persistent matrix of the tooth being attached to the peri- 

 osteum, and protected by the contiguous soft parts. In all the 

 Rodents with the wide antorbital vacuities, the fore part of the 

 masseter takes its origin from the facial bones anterior thereto, 

 and traverses the vacuity in its oblique course beneath the fore 

 part of the zygoma, to expand and blend with the normal part of 

 the masseter. 



The lower jaw is modified for the lodgement of the pair of 

 long, curved, scalpriform incisors, the sockets of which may 

 extend to the middle (Hare) or even to the hind part (Beaver, 

 Porcupine) of the ramus : in the latter case the prominent inner 

 wall curves beneath the molar alveoli and forms, as in figs. 238, 

 241, 242, c, the lower part of the horizontal ramus. The condyle, 

 crowning this, rises usually high above the grinders ; it is lowest 

 in the Capybara and some Cavies : in all Rodents the condyle 

 is convex transversely and extended longitudinally. The chief 

 work of the teeth being by horizontal movements to and fro, all 

 that part of the ascending ramus serving for the implantation of 

 the masseter is expanded, while that for the temporal muscle is 

 reduced, so that the ^ coronoid ' process is very small, and may 

 be a mere tubercle {Lagomi/s), while the angle of the jaw usually 

 forms the whole base of the ascending ramus, projecting beloAV 

 its fore part, angularly in the Hare, fig. 233, a ; and behind its 

 back part, extensively in Cavies, fig. 234, «, and Voles, fig. 

 241, a. In most of these it is long and pointed; but is obtuse 

 and compressed in Dolichotis : it is subquadrate in Squirrels, 



