RODENTIA. 



375 



palate, of which it occupies rather less than a 

 third. Thejugal bone reaches to the anterior 

 base of the zygomatic arch, where it articulates 

 with the lachrymal as well as with the maxil- 

 lary bone ; it is connected with the zj gomatic 

 apophysis of the os temporis by a horizontal 

 suture, which occupies all the second half of 

 the arch, so that it extends as far back as the 

 glenoid cavity, the external margin of which it 

 fills. The lachrymal is of moderate extent in 

 the orbit, but is scarcely visible beyond the 

 margin of that cavity ; besides its canal, 

 which is altogether within the orbit, there is a 

 small unossified space between it and the 

 maxillary bone, situated very near to the pos- 

 terior opening of the sub-orbital canal. The 

 large space occupied by the maxillary in 

 the orbit keeps the lachrymal widely sepa- 

 rated from the palatine bone, with which it 

 articulates so extensively in the Carnivora. 

 The palatine bone occupies, posteriorly, 

 about one-fifth of the extent of the palate. 

 After having formed the root of the pterygoid 

 alas, it is prolonged between them for about 

 half their length laterally ; it mounts up into 

 the temporal region as high as just beneath 

 the optic foramen ; it there spreads out back- 

 wards as far as the spheno-orbital foramen, 

 and forwards, as the foramen which represents 

 the spheno-palatine. The internal pterygoid 

 process is not detached from the sphenoid, 

 and terminates posteriorly in a long hook. The 

 external pterygoid plate is very distinct ; 

 although but little prominent, it covers the 

 vidian foramen, and touches with its point the 

 extremity of the maxillary. 



In the squirrel, the lachrymal hook is formed 

 by the bone of that name ; but it is also 

 doubled by a similar unciform process, derived 

 from the jugal. There is no membranous 

 space between the lachrymal and the maxillary. 

 The prolongations of the palatine bone be- 

 tween the pterygoid u\as are shorter. In other 

 respects the relations of the bones to each 

 other are very similar to what exists in the 

 marmot. 



Fig. 256. 



Skull of the Beaver (Castor Fiber}. 



In the beaver, the post-orbital apophysis of 

 the os malae is very large and blunt, and all 

 this portion of the bone very broad ; it occu- 

 pies the greater portion of the zygomatic arch. 

 The two nasal bones are broader in their 

 middle, and both the intermaxillary and max- 



illary bones reach up as far as the frontals. 

 The lachrymals are small, especially that por- 

 tion of them which is situated without the 

 orbit, to which the jugal bones touch. The 

 vaulted portion of the maxillary bone is very 

 extensive and well circumscribed in adult ani- 

 mals ; on its external margin, by the ridge, 

 which is con'inuous with the inferior edge of 

 the zygomatic arch, and internally by another 

 ridge, which commences close to the sub- 

 orbital foramen, and mounts up on the cheek 

 to join the ridge last mentioned. The pala- 

 tine bone occupies in the palate a triangular 

 space, extending as far forward as opposite 

 the second molar tooth; it terminates pos- 

 teriorly between the two pterygoid alae. The 

 external pterygoid apophysis is of moderate 

 length, nearly rectangular in its shape, and is 

 pierced at its base by the vidian canal : it ar- 

 ticulates broadly with the posterior part of the 

 maxillary in such a way as to exclude the 

 palatine both from the orbit and from the 

 temple. The internal pterygoid apophysis is 

 of a hooked form, the point of the hook 

 reaching as far as the tympanum. 



In the oryctcres, the jugal bone com- 

 mences at about the anterior fourth of the 

 length of the zygomatic arch, and conse- 

 quently remains widely separated from the 

 lachrymal. The ossa nasi constitute scarcely 

 half the breadth of the snout, in which the 

 maxillary occupies much less space, it being 

 here the inter-maxillary which principally 

 forms it. The last-mentioned bones mount 

 up upon the forehead higher than the bones 

 of the nose a circumstance which is the 

 reverse of what occurs in the beaver. The 

 concavity of the maxillary beneath the base of 

 the zygomatic arch is reduced to a slight oval 

 depression; but its zygomatic apophysis is 

 very long ; it is the maxillary bone and the 

 frontal, to which it is joined by a long suture, 

 which forms almost alone the osseous walls of 

 the orbit. There is no lachrymal suture visible, 

 although the lachrymal canal is distinct enough. 

 The external pterygoid apophysis presents 

 neither crest nor prominent angle ; the inter- 

 nal resembles that of the beaver. 



In the ondatra and the water voles, the 

 bones of the nose, which are pointed at their 

 summits, are considerably enlarged at their 

 inferior extremities. The intermaxillaries 

 occupy a smaller portion of the snout than 

 the preceding pieces, the oblique excavation 

 at the root of the zygomatic arch exists ; but 

 it is separated from the cheek superiorly by the 

 vertical prolongation of the sub- orbital fora- 

 men. The malar apophysis of the maxillary 

 extends beneath the jugal until it almost 

 reaches that of the temporal ; so that the jugal 

 is only free at its lower margin for a very 

 small space, and is very far removed from the 

 lachrymal, which latter bone does not appear 

 external to the orbit, it being concealed in the 

 sub-orbital canal. The os palati extends into 

 the palate as far as the first molar tooth, but 

 is not visible either in the orbit or in the 

 temple, in which latter region the maxillary is 

 connected to the two sphenoids and to the 



13 B 



