140 MAMMALIAN DESCENT. [LECT. V. 



Very soon, whilst the superficial bone is forming by 

 transformation of the superficial cartilage, a thick, solid 

 bar of bone is formed in the front third of the Meckelian 

 rod of the embryo Hedgehog. The upper part of the 

 cerato-branchial bar becomes detached after a time, but 

 not until it has become ossified ; this ossification is 

 arrested, to form the malleus (or hammer bone) of the 

 middle ear. The huge epibranchial, or upper jaw of the 

 Shark, is represented in the Hedgehog by two tracts of 

 cartilage, one small and the other large. 



The large cartilage is the hinge part the hinder 

 region of the upper jaw. The fore part, which in the 

 Shark carries the upper teeth, is represented by an oval 

 segment of solid hyaline cartilage, which becomes con- 

 verted into the hamular process of the pterygoid 

 bone; the larger hind piece becomes the incus, or 

 / anvil. The stapes is stirrup-shaped, and is, as I have 

 before stated, the pharyngo-branchial element of the 

 perfect hyoidean (or second arch). There is a ring, 

 partly cartilaginous and partly bony, formed round the 

 interspace (cleft, or tympanic cavity) of these two arches. 

 The inner part is bony, and forms the annulus, or osseous 

 ring for the ear-drum ; the next is a partly segmented 

 series of cartilaginous annuli or rings forming the coating 

 of the meatus externus or ear-porch, which ends in the 

 concha or projecting part of the ear. 



The whole of this latter structure is a specialisation of 

 the familiar " spiracular ray" of the Shark that small 



