LECT. V.] EAR-DRUM OF HEDGEHOG. 13 It 



these cartilages become flattened and very solid, much 

 more like their counterparts in the Shark than what is 

 seen in the more specialised forms of fishes. I have 

 followed the changes that take place in these parts, 

 through about nine stages, for a lesser number would 

 not have given me all I wanted in searching after the 

 meaning of this transformation. Before this rod be- 

 comes ossified, a thin superficial plate of bone, attached 

 to, and grafting itself upon, a thick superficial slab of 

 cartilage, appears above and outside the lower two- 

 thirds of Meckel's cartilage. The bone is the well- 

 known dentary of ganoid and bony fishes ; the slab of 

 outer cartilage answers to the small lower labial of a 

 common Shark, and to the huge massive lower labial 

 of a Chimsera. I have repeatedly shown that the upper 

 and lower jaw of those kinds of fishes is formed by the 

 bending of the first internal gill-arch over the cavity 

 of the mouth. The upper jaw, then, of a Shark, is, in 

 technical language an "epi-branchial" element, the lower 

 jaw is the " cerate-branchial" of the same first post-oral 

 arch. Above the Sharks and Skates, the joints or seg- 

 ments of the gill-arches become ossified, and each piece 

 is further segmented into two, so that above the epi- 

 branchial we have a " pharyngo-branchial," and below 

 the cerate-branchial there is generally a " hypobran- 

 chial." These further subdivisions we may forget for 

 the present ; they are very inconstant in the first and 

 second arches of the throat. 



