Branchiostegal rays. — Six in number; increasing 

 only slightly in length posteriorly in the series. The 

 branchiostegal rays are mostly flat plates of decreasing 

 width posteriorly in the series and articulate anteriorly 

 by fibrous tissue variously to the posterior end of the 

 ceratohyal and the lower lateral surface of the epihyal, as 

 described above. The decreasing width of the rays pos- 

 teriorly in the series makes the last few rays somewhat 

 rodlike rather than flat plates. 



Urohyal. — Laterally expanded and thickened 

 along its anterior edge and, to a lesser extent, for a short 

 distance anterodorsally. Otherwise a flat, thin plate ar- 

 ticulating by fibrous tissue anterodorsally primarily with 

 the undersurfaces of the ventral hypohyals. 



Branchial Arches. — All of the elements are cartilage - 

 filled at their edges of articulation with the other 

 elements of 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 four pairs of pharyngo- 

 branchials. Four gills are present, with a slit between the 

 fourth arch and the lower pharyngeal. 



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

 pharyngobranchial elements present. First basibranchial 

 the shortest of the three basibranchial elements, laterally 

 compressed in the anterior half of its length, where it is 

 held by fibrous tissue between the medial edges of the 

 dorsal hypohyal, while posteriorly it articulates mainly 

 with the anterior end of the second basibranchial and 

 only secondarily posterolaterally with the first hypo- 

 branchials. First hypobranchial about the same size as 

 the second hypobranchial but more rectangular in form; 

 articulates ventrally with an indented region along the 

 lateral surface of the anterior half of the second basi- 

 branchial. First ceratobranchial slightly the longest of 

 the ceratobranchial elements, which decrease slightly in 

 size or length posteriorly in the series; a more or less 

 columnar shaft of bone without a prominent ventral keel. 

 First epibranchial a more or less rectangular block of 

 bone except distally where it slightly bifurcates into two 

 short projections; the anterior projection articulating 

 with the base of the first pharyngobranchial, or suspen- 

 sory element, and the posterior projection articulating 

 with the second pharyngobranchial. First pharyngo- 

 branchial (suspensory pharyngeal) a short sturdy rod of 

 bone articulating ventrally with the anterior projection of 

 the first epibranchial and dorsally by fibrous tissue to the 

 lateral surface of the ventral keel of the parasphenoid in 

 the region just below the articulation of the anteroven- 

 tral edge of the prootic with the parasphenoid. 



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

 pharyngobranchial elements present. Second basi- 

 branchial slightly the longest of the three basibranchial 

 elements, with concave lateral surfaces in the anterior 

 half of its length for articulation with the ventral ends of 



the first hypobranchials; articulates anteriorly with the 

 first basibranchial, posteriorly with the third basi- 

 branchial and posterolaterally with the second hypo- 

 branchials. Second hypobranchial expanded ventro- 

 medially where it articulates with the posterior end of the 

 second basibranchial and the anterior end of the third 

 basibranchial; articulates dorsally with the second 

 ceratobranchial, which in turn articulates dorsally with 

 the laterally expanded basal portion of the second epi- 

 branchial. The second pharyngobranchial is the first of 

 the tooth bearing pharyngobranchials, having six or 

 seven teeth, set in deep sockets, in a single row along its 

 ventral edge. The teeth are laterally compressed and 

 taper to blunt points distally and are much smaller than 

 those in the jaws. They are replaced by teeth which 

 develop in new sockets just anterior to the sockets of the 

 old teeth, much in the same manner as in the replace- 

 ment of the teeth in the jaws. 



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

 pharyngobranchial elements present. Third basi- 

 branchial articulates anteriorly with the posterior end 

 of the second basibranchial, anterolaterally with the 

 second hypobranchials, posterolaterally with the third 

 hypobranchials, and posteriorly, more distantly, with the 

 fourth ceratobranchials. Third hypobranchial with an 

 anteroventral process which articulates by fibrous tissue 

 with the posterodorsal edge of the urohyal; at its postero- 

 dorsal end it articulates medially with the posterior end 

 of the second basibranchial, anterolaterally with the 

 end of the third ceratobranchial. Third ceratobranchial 

 articulates dorsally with the third epibranchial, which is 

 about equal in size to the second pharyngobranchial. 

 Third pharyngobranchial bears six to seven teeth in a 

 single row along its ventral edge, similar to those on the 

 second pharyngobranchial. The three tooth bearing 

 pharyngobranchial elements (second to fourth) are more 

 or less closely held to one another by fibrous tissue. 



Fourth arch. —Cerato-, epi-, and pharyngo- 

 branchial elements present. In the absence of a fourth 

 basibranchial and fourth hypobranchial, the fourth 

 ceratobranchial articulates ventrally with the posterior 

 ends of the third basibranchial and hypobranchials. 

 Fourth epibranchial a more or less rodlike element, 

 slightly expanded ventrally; articulates ventrally with 

 the fourth ceratobranchial and dorsally with the small 

 fourth pharyngobranchial. The fourth pharyngo- 

 branchial is the smallest of the three tooth bearing 

 elements, and bears four or five teeth in a single row 

 along its ventral edge, these teeth being smaller than 

 those on the second and third pharyngobranchials but of 

 similar shape. 



Fifth arch. —Ceratobranchial (lower pharyngeal) 

 element only. Fifth ceratobranchial wider in the middle 

 than anteriorly where it is generally rounded or pos- 

 teriorly where it tapers to a short shaft, bearing teeth 

 more or less placed in three rows, with those of the pos- 

 terior row the largest. All of the teeth tend to be more 



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