B4 
5.—Hyorp Arcu* (Fig. 6). 
Ocular Side. 
Uro-hyal (U.Hy.).t—A short bony zod but car- 
tilaginous in front and behind, and articulating with the 
upper hypo-hyals and slightly with the first basi-branchial. 
Hypo-hyals (/7.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 (/p.Hy.).—Also double, the 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,t but may be 
seen on holding the hyoid bar up to the hght. 
*Tt 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 any one 
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-segment. In the latter case the terms basi- and hypo- are 
synonyms (and are used indifferently) ; in the former, they are not. 
+ 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. 
{ Shufeldt, op. cit., p. 819, and fig. 32. 
