34 



5.— HYoin Arch* (Fig-. 6). 

 Ocular Side. 



Uro-hyal {U.Hy.).-f — A short bony rod but car- 

 tilaginous in front and behind, and articulating with the 

 upper hypo-hyals and slightly with the first basi-branchial. 



Hypo-hyals {H.Hy.). — Two pieces, as in the Cod, 

 partly of bone and partly of cartilage and loosely articu- 

 lating in the middle line with the same elements of the 

 other side. The upper one is perforated in the middle, 

 thus giving a false impression as to the whereabouts of the 

 suture between the two. 



Cerato-hyal {C.Hy.). — A stout bar traversed in front 

 (anterior face) by a longitudinal groove, the texture of the 

 bone on each side of which running in different directions, 

 thus seeming to indicate that the cerato-hyal, as well as 

 the hypo- and epi-hyals, is either splitting or has been 

 formed by fusion. 



Epi hyals {Ep.Hy.). — Also double, ^he lower piece 

 being entirely cartilaginous and the upper partly so. The 

 suture between the cerato- and upper epi-hyal is obscured 

 by an overgrowth of bone as in Micropterus,+ but may be 

 seen on holding the liyoid bar up to the light. 



* It is here necessary to explain the precise significance of the prefixes 

 basi- and hypo- as applied to parts of the visceral skeleton. The term basi- 

 can only be applied to a median unpaired ventral element, and the term 

 hypo- to the pair (i.e., one on each side), immediately succeeding it. Now 

 whilst these three elements may, and often do, exist side by side in anyone 

 species, the basi- element may be absent, and a median unpaired ventral 

 piece formed by the two hypo- elements fusing together, the result being a 

 secondary basi-segraent. In the latter case the terms basi- and hypo- are 

 synonyms (and are used indifferently) ; in the former, they are not. 



f The terms basi-hyal, glosso-hyal, ento-glossal and basi-branchiostegal 

 have also been applied to this bone in Fishes by different authors, and the 

 same terms, or some of them, have been applied to other elements in 

 higher Vertebrates. The synonymy is too complex to be discussed here. 



I Shufeldt, oiJ. cit., p. 819, and fig. 32. 



