CASTOR 667 



cavity of the auditory bullee undivided by osseous septa. They 

 also do not show the marked increase in the relative size of the 

 brain characteristic of so many Sciuridce. 



On the other hand they have, in many respects, attained a 

 far higher degree of specialisation than that manifested by 

 Sciurid(s. The incisor teeth are enormously developed, and 

 the peculiar rodent function of gnawing reaches in this family 

 its highest expression. In adapting themselves to a hard, 

 coarse vegetable diet, their cheek-teeth have become markedly 

 hypsodont, and display, when the tubercular caps are removed 

 by wear, a pattern of deeply re-entrant, transverse enamel folds. 

 Only one premolar is present in the upper jaw on each side, 



the dental formula being consequently i—, P ~, m-^ = 2o; and 



the cheek-teeth are arranged in anteriorly convergent rows. 

 The temporal, masseter, and pterygoid muscles are all power- 

 fully developed, their strength being betrayed in the skull by 

 heavily-built zygomatic arches, a moderately salient sagittal 

 crest and deep pterygoid fossae, the latter becoming, as usual, 

 deeper and more extensive in proportion as the angular pro- 

 cesses of the mandible diminish in size. Except in the very 

 highly specialised extinct genus Castoroides, from the Pleisto- 

 cene of North America, the jugal articulates with the lachrymal, 

 and the fibula remains distinct from, though often closely 

 connected with, the tibia, both being features shared with the 

 Sciuridcs. 



The British members of the family belong to two genera — 

 Castor, now locally extinct, though surviving as a mere 

 remnant in parts of Europe and Asia, as well as in North 

 America ; and Trogontherium, now wholly extinct, and known 

 only from the Pliocene and Pleistocene of Europe and Asia. 



Genus CASTOR. 



1758. Castor, Carolus Linnaeus, Syst. Nat., lothed., i., 58 ; type C. fiber, selected by 



tautonymy. 

 1806. Fiber, Dumeril, Zoologie Analytique, 18; a substitute for Castor; nee Fiber 



(Cuvier, iScxs), which is a synonym of Ondatra, Link. 



This genus has a circumpolar distribution, and comprises 

 the Beavers of the Old and New Worlds. It evinces in a high 



