BRANCHIAL APPARATUS. 



Hyoid Arch, Branchiostegal Rays, and Urohyal. 



Hypohyals. — Both hypohyal elements well de- 

 veloped; ventral element larger than the dorsal ele- 

 ment; dorsal element cartilage filled along its posterior 

 and ventral edges, the ventral element cartilage filled 

 along its posterior and dorsal edges. The two elements ar- 

 ticulate with one another and with the ceratohyal 

 through cartilage; the anteromedial edges of both 

 elements articulate by fibrous tissue with their opposite 

 members. The dorsomedial edge of the dorsal hypohyal 

 articulates by fibrous tissue posteriorly with the first 

 basibranchial and anteriorly with the reduced urohyal, 

 while the dorsomedial edge of the ventral hypohyal also 

 articulates by fibrous tissue with the urohyal. 



Ceratohyal. — A wide flat plate; shortened antero- 

 posteriorly and expanded dorsoventrally to such an ex- 

 tent that the former dimension is shorter than the latter; 

 cartilage filled along all of its edges except at the in- 

 dented regions posterodorsally and anteroventrally; ar- 

 ticulates through cartilage anteriorly with the ventral 

 hypohyal, anterodorsally with the dorsal hypohyal and 

 posteriorly with the epihyal. The first two branchios- 

 tegal rays articulate by fibrous tissue with slight depres- 

 sions on the ventral edge of the ceratohyal at the an- 

 terior edge of its deeply indented anteroventral region. 

 The next three branchiostegal rays articulate by fibrous 

 tissue with the posteroventral edge of the ceratohyal, and 

 the last ray with the ventral edge of the area of ar- 

 ticulation between the epihyal and ceratohyal. 



Epihyal. — Large, elongate dorsoventrally; cartilage 

 filled at its anterior, anterodorsal and ventral edges; ar- 

 ticulates through cartilage anteriorly with the cerato- 

 hyal, while posterodorsally it supports the interhyal by 

 fibrous tissue. Just anterior to its articulation with the 

 interhyal, the lateral surface of the epihyal articulates by 

 fibrous tissue with the posterior end of the interoper- 

 culum. 



Interhyal. —A short, slender rod; cartilage filled at 

 both ends; articulates by fibrous tissue ventrally with the 

 epihyal and dorsally with the slight concavity on the ven- 

 tral edge of the metapterygoid immediately behind the 

 posterodorsal end of the symplectic. 



Branchiostegal rays. —Six in number; the first ray 

 shorter than the second; the third to fifth rays of about 

 the same length as the first ray; the sixth ray by far the 

 longest. The rays articulate by fibrous tissue to the 

 ceratohyal and, to a lesser extent, the epihyal, as ex- 

 plained under those bones. 



Urohyal. -—Reduced to a short depressed plate 

 without a ventral keel; articulates by fibrous tissue 

 anterodorsally with the anteromedial edge of the dorsal 

 hypohyal and dorsomedial edge of the ventral hypohyal; 



articulates by fibrous tissue posteriorly with the ventral 

 surfaces of the first two basibranchial elements. 



Branchial Arches. — All the elements are cartilage 

 filled at their edges of articulation with the other 

 elements in the series, and the articulations are usually 

 through cartilage and fibrous tissue. The branchial 

 arches are composed of three basibranchials, three pairs 

 of hypobranchials, five pairs of ceratobranchials, four 

 pairs of epibranchials, and two (sometimes three) pairs 

 of pharyngobranchials. Four gills are present, with a 

 small slit between the fourth arch and the lower pharyn- 



First arch. — Basi-, hypo-, cerato-, epi-, and, 

 sometimes, pharyngobranchial elements present. First 

 basibranchial short, slightly wider posteriorly than an- 

 teriorly; displaced forward so that it articulates pos- 

 teriorly with the second basibranchial and anteriorly 

 with the hypohyals and urohyal, but with no direct con- 

 nection with the first hypobranchials. First hypo- 

 branchial wide dorsally but very narrow ventrally; the 

 largest of the hypobranchial elements, which decrease in 

 size posteriorly in the series; articulates ventrally with 

 the lateral surface of the middle region of the second 

 basibranchial and dorsally with the first cerato- 

 branchial. First ceratobranchial a long sturdy com- 

 pressed rod; the longest of the ceratobranchial elements, 

 which decrease in length posteriorly in the series; from 

 the first to the fourth ceratobranchial the ventral regions 

 become increasingly enlarged (the degree of compression 

 of the rods, and hence their width, appears to increase 

 somewhat with increasing specimen size); no ventrally 

 directed flange present on any of the ceratobranchials; 

 articulates ventrally with the first hypobranchial and 

 dorsally with the first epibranchial. First epibranchial a 

 short rod, somewhat narrowed in the middle; articulates 

 dorsally with the anterolateral surface of the second 

 pharyngobranchial and with the base of the first or 

 suspensory pharyngobranchial when such is present; the 

 shortest of the epibranchial elements, which increase 

 slightly in size posteriorly in the series. First 

 pharyngobranchial absent, at least as an ossification, in 

 the majority of the specimens examined, but present in 3 

 out of 13 specimens as a short slender toothless rod whose 

 distal end is held by fibrous tissue to the medial surface 

 of the extreme anterior end of the ventral flange of the 

 prootic shelf just lateral to the region where the prootic 

 interdigitates with the laterally expanded portion of the 

 parasphenoid in the anterior region of the orbit. 



Second arch. — Basi-, hypo-, cerato-, epi-, and 

 pharyngobranchial elements present. Second basi- 

 branchial the longest of the basibranchials, the first and 

 third being of about the same length, but of different 

 widths; articulates anteriorly with the first basi- 

 branchial, laterally with the first hypobranchials, and 

 posteriorly with the third basibranchial. Second hypo- 

 branchial somewhat wider dorsally than ventrally; ar- 

 ticulates ventrally with the area of articulation between 



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