387 

 2146. The right humerus. 2147. The right ulna. 



2148. The right radius. 2149. The right os innominatum. 



2150. The right femur. 2151. The right tibia. 



2152. The right fibula. 2153. The left tibia, longitudinally bisected. 



2154. The left fibula. 2155. The right astragalus. 



2156. The right calcaneum. 



Family Castorida (Beavers, Voles). 

 Genus Castor. 



Dental formula : i j=|, p j=i, m ^=20. 



2157. The skeleton of a young Canada Beaver (Castor canadensis). 



The vertebral formula is : 7 cervical, 1 9 dorse-lumbar, 4 sacral, and 25 caudal. The acces- 

 sory process appears upon the seventh dorsal vertebra, and divides at the eleventh into 

 anapophysis and metapophysis : the anapophysis disappears on the last lumbar. The last 

 true molars (m 3) have recently been acquired, but the milk-molars are not shed : the germ 

 of the premolar is exposed on the right side of the lower jaw. The elements of the occipital 

 bone are still unanchylosed : the lower third of each condyle is formed by the basioccipital, 

 the under surface of which presents a large and deep excavation. The upper part of the 

 foramen magnum is completed by the broad superoccipital. The mastoid is larger than in 

 the Porcupines, and articulates anteriorly with both the parietal and squamosal : it is anchy- 

 losed to the petrosal. There is a perforation in the suture between the superoccipital and 

 mastoid. The interparietal is large, and wholly upon the upper surface of the cranium. 

 The squamosal is perforated behind and below the root of the zygoma. The frontals are 

 small and almost flat above. The nasal bones extend a little further back than the premax- 

 illaries, but not beyond the transverse line which extends between the antorbital tuberosities. 

 The anterior root of the zygoma formed by the maxillary is a simple plate which appears to 

 be imperforate, the orifice of the slender antorbital canal being concealed by a vertical ridge 

 of the maxilkry, which inclines forwards over the maxillo-premaxillary suture. 



Mus. Brookes. 

 3 D 2 



