CASTOR 667 
cavity of the auditory bullz undivided by osseous septa. They 
also do not show the marked increase in the relative size of the 
brain characteristic of so many Sczuride. 
On the other hand they have, in many respects, attained a 
far higher degree of specialisation than that manifested by 
Sceurtde. 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=, o, m+ =203 and 
o 
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 fossz, 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 Cas¢oroides, 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 
Sciuride. 
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 Zvogonthertum, now wholly extinct, and known 
only from the Pliocene and Pleistocene of Europe and Asia. 
GENUS CASTOR. 
1758. CASTOR, Carolus Linnzus, Syst. Wat., 10th ed., i., 58 ; type C. fiber, selected by 
tautonymy. 
1806. FIBER, Dumeril, Zoologie Analytique, 18; a substitute for Castor; nec Fiber 
(Cuvier, 1800), 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 
