Lect. v.] EAE-DEUM OF HEDGEHOG. 139 



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 " cerato-branchial" of the same first post-oral 

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

 ments of the giU-arches become ossified, and each jDiece 

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

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

 the cerato-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. 



