154 MAMMALIAN DESCENT. [Lkct. VI 



angulare, aiul the angulare of the ovipara. More tlian 

 this, a sickle of l)one j^'^sses across in front of the tym- 

 panic cavity, exactly as in the Marsupials. But thi.s 

 l^retympanic bar is soon absorbed, and so, after a time, 

 is the rest of the wild growth of fibrous bone, until, in 

 the adult, the processus gracilis, or slender process of 

 the hammer bone, is a mere point. In the Crocodile the 

 cavum tympani (drum cavity) is mainly formed 1»y the 

 hollowing out of the quadratum (or jaw-pier). In the 

 Bird it is behind that bone, which, however, is hollow, 

 and opens into the cavity by a considerable passage. 

 In the Mole, the incus, malleus, and stapes (the small 

 ear-bones), lie in the drum cavity, and like their 

 counterparts in the Bird — the quadratum, articular Ijone 

 and columella (stapes) — are all pneumatic ; little, hollow 

 shells of bone, opening into the general cavit}^ In some 

 adult Insectivora the sheath of the stapedial artery, which 

 runs from the common carotid to the artery of the 

 lower jaw, becomes ossified, and as this passes through 

 the foot-h(»le of the stirrup, that bone is fastened in its 

 place, the little bony tube being continuous with the os 

 petrosum (or stony bone of the inner ear) at each end. 

 This curious state of things is only temporary in the 

 Mole ; in young specimens, three parts grown, it is seen, 

 Ijut this tubular rod becomes absorbed afterwards. As 

 in Birds and Crocodiles, the adult Mole's skull is 

 extremely pneumatic, or full of air cavities. We only 

 retain the mastoid cells in addition to the typanic 



