374- 



RODENT1A. 



face in the occipital region of the skull, is ex- 

 tremely dilated, insomuch, indeed, that this 

 dilatation appears in the upper wall of the 

 skull, in the shape of two large projections, 

 bounded in front by the parietals, internally 

 by a plate common to them and the occipital 

 bone, and posteriorly by a long narrow trans- 

 verse projection from the occipital, which is 

 in contact with the petrous bone, and exter- 

 nally by another thin and pointed slip, which 

 forms the posterior termination of the tempo- 

 ral bone, and which projects above the audi- 

 tory meatus to join that derived from the 

 occipital. We have seen, above, that in the 

 jerboa a similar disposition exists. 



Bones of the face. In the Rodentia the 

 intermaxillary bones are of enormous dimen- 

 sions, on account of the great size of the inci- 

 sor teeth, so that the maxillary bones are 

 pushed very far backwards ; these latter form 

 a large portion of the inner wall of the orbit, 

 into the composition of which the os palati 

 enters but slightly, and sometimes, indeed, 

 not at all. The anterior boundary of the 

 orbit is formed by a process of the maxillary 

 bone, which proceeds to contribute to the 

 formation of the zygomatic arch in such a 

 way that the os malae is, as it were, suspended 

 in the centre of the arch between the apophy- 

 sis, derived from the maxillary and the zygo- 

 matic process of the temporal bone.* It 

 joins neither the frontal nor the sphenoid. 

 The elongation of the ossa nasi is such that 

 the opening of the nose is situated quite at 

 the extremity of the snout. 



In the aye-aye the bones of the nose are 

 short and broad. The intermaxillaries mount 

 up along their sides by a broad process, which 

 occupies part of the snout, and are articulated 

 to the frontal by a tolerably broad space ; 

 they touch, likewise, the lachrymals which 

 encroach upon the cheek ; while the canal 

 situated between the latter bones, the maxil- 

 lary, and the jugal, is out of the orbit. The 

 jugal apophysis of the maxillary arises oppo- 

 site the second molar tooth, and the boundary 

 of the jugal bone is placed at the anterior 

 base of the zygomatic arch. It articulates with 

 the lachrymal, both within and without. The 

 orbit is very broad, and furnishes a large post- 

 orbital apophysis, which joins that derived 

 from the frontal bone. The palatine bone 

 advances but a little way into the palate, ter- 

 minating by a straight suture between the 

 last molar teeth. The palatine portion of 

 the pterygoid alse is simple ; their sphenoidal 

 portion is divided into two laminae, the ex- 

 ternal of which is prolonged as far as the 

 tympanum, to which it is articulated, as well 

 as to the inner border of the glenoid surface. 

 In the temporal region, the palatine bone re- 

 mains behind the posterior margin of the 

 maxillary, between the latter bone and the 

 two sphenoids, only touching the frontal by 

 its apex. 



In the hares, the intermaxillary bone pre- 



* It will be seen from the details that follow, that 

 the part played by the os malae in the construction 

 of the cheek is not always so simple. 



sents, besides its palatine portion, which is 

 large, a long ascending apophysis, which is at 

 first imbedded between the maxillary and the 

 os nasi, and subsequently between the latter 

 and the apophysis of the frontal, to which 

 latter it is connected. All that portion of the 

 maxillary bone which forms the cheek is, in 

 the adult animal, riddled with holes, so as to 

 have the appearance of lace-work. The la- 

 chrymal in the orbit is tolerably large ; exter- 

 nally, it gives off a blunt hook, beneath which 

 is the lachrymal canal, situated upon the very 

 edge of the orbit. The zygomatic portion 

 of the maxillary bone is short ; its inferior 

 margin forms a ridge, which projects slightly 

 externally, and presents a flattened surface, 

 from which arises one of the portions of the 

 masseter muscles. It is this surface which 

 we shall see in other % Rodentia become 

 rounded into a more or less oblique vaulted 

 space, and in others become transformed into 

 a wide ring. The union between the maxil- 

 lary and the jugal bones is so soon obliterated, 

 that unless we examine very young indivi- 

 duals we should be tempted to believe that no 

 jugal existed. This latter bone is arched in- 

 feriorly, and extends by means of a process 

 beneath the zygomatic portion of the temporal 

 bone. Besides the floor with which it covers 

 the roots of the teeth, the maxillary gives off 

 a narrow plate, which mounts into the orbit 

 as high as the os frontis, between the lachry- 

 mal, from which, however, it is separated by a 

 membranous space and the anterior sphenoid. 

 The vomer is visible at the hinder part of the 

 septum, which separates the foramina inci- 

 siva. The palatine occupies beneath the an- 

 terior sphenoid in the orbit a much greater 

 space than in other Rodents ; inferiorly it 

 extends as far as the third molar tooth, and is 

 deeply indented as far as the fourth. The ptery- 

 goid alee extend to the azygos portion, oc to the 

 body of the anterior sphenoid, but they are 

 separated from that of the posterior sphenoid 

 by a membranous space on each side. The 

 posterior sphenoid has on each side two 

 pterygoid plates, which are both of them con- 

 tiguous to those of the palate bones ; the in- 

 ternal ones terminate in a slender point or 

 style. 



In the lagomys, the base of the zygomatic 

 arch gives off a process, which is directed 

 downwards ; and the jugal bone, after having 

 passed beyond the zygomatic process of the 

 temporal, is prolonged directly backwards into 

 a lengthened point. 



In the marmot, the two nasal bones con- 

 stitute the middle of the upper vault of the 

 snout. On each side of them the ascending 

 apophysis of the intermaxillary bones, which 

 are broader than in the hares, run up to be 

 articulated with the frontal, the anterior 

 border of which is transverse and only 

 slightly festooned. The external surface of 

 the maxillary is concave beneath a ridge, which 

 is continuous with that of the zygomatic arch, 

 extending as far as the intermaxillary suture. 

 Setting off from this point, the intermaxillary 

 suture descends vertically to embrace the 



