393 



2200. The left humems of a Beaver (Castor canadensis), vertically bisected, to show 

 the absence of the medullary cavity, its place being occupied by a coarse can- 

 cellous tissue. Presented by Prof. Owen, F.B.S. 



2201. The right femur of a Beaver (Castor canadensis), vertically bisected, to show 

 the absence of the medullary cavity, its place being occupied by a coarse can- 

 cellous tissue. Presented by Prof. Oioen, F.E.S. 



2202. The left tibia with the anchylosed fibula of a Beaver (Castor canadensis), 

 vertically bisected; showing the absence of a medullary cavity in the tibia, 

 and the complete confluence of its compact wall with the anchylosed part of 

 the fibula. Presented by Prof. Owen, F.R.S. 



2203. The right ramus of the lower jaw of a Beaver, in which the incisor, by want 

 of opposition from that in the upper jaw, has proceeded to grow until its 

 point has again penetrated the mouth between the condyloid and coronoid 

 processes, as far as the base of its own socket, having described a complete 

 circle, but a little obliquely. The atrophied condition of the articular process 

 and ascending ramus of the jaw indicates the impediment to its movements 

 which this anomalous growth of the incisor has occasioned. Hunterian. 



2204. The dried tail of a Beaver (Castor canadensis), showing the scaly integument. 



Hunterian. 

 Genus Fiber. 



Dental formuk : t {, p j=|, m |Ef = 16. 



2205. The skull of the Ondatra or Musk Vole (Fiber zibeticus). 



The basioccipital is not excavated, as in the Beaver, but there is the same perforation 

 between the mastoid and superoccipital, and a large vacuity in the posterior process of the 

 squamosal communicating directly with the cranial cavity. The squamosal is unusually 

 expanded above the zygomatic process, and articulates largely with both frontal and parie- 

 tal. The zygomatic process of the maxillary reaches almost to that of the squamosal, and 

 supports a great part of the malar bone. The antorbital foramen is larger than in the Beaver, 

 but is bounded externally, as in it, by a nearly vertical ridge of the maxillary. The interor- 

 bital septum is perforated behind, beneath the orbitosphenoid. There is no distinct lacrymal 



3 E 



