held to the lower posterolateral surface of the ceratohyal, 

 while the fourth is held to the posterior edge of the 

 ceratohyal and to the cartilage between the epihyal and 

 ceratohyal. The fifth and sixth branchiostegal rays are 

 held to the lateral surface of the epihyal; the sixth 

 branchiostegal is the longest of the branchiostegal rays. 

 All of the branchiostegal rays articulate with the 

 ceratohyal or epihyal by fibrous tissue. 



Urohyal. — Thin and shallow; laterally expanded 

 and wider than long; articulates by fibrous tissue 

 posterodorsally with the anterior edge of the first basi- 

 branchial and anterodorsally with the dorsomedial sur- 

 face of the dorsal hypohyal; in the larger specimen the 

 urohyal is even more laterally expanded than as illus- 

 trated for the smaller specimen. 



Branchial Arches. — All 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 basi- 

 branchial elongate, slightly compressed in the middle; 

 displaced forward so that it articulates posteriorly with 

 the second basibranchial and only posterolaterally with 

 the first hypobranchials. First hypobranchial an elongate 

 rod; articulates with the anterior end of the second basi- 

 branchial and the posterior end of the first basi- 

 branchial. First ceratobranchial a narrow rod; slightly 

 shorter than the second ceratobranchial but of about the 

 same length as the third and fourth ceratobranchials; ar- 

 ticulates ventrally with the first hypobranchial and dor- 

 sally with the first epibranchial. First epibranchial 

 rodlike ventrally but becoming wider and flatter dorsal- 

 ly; the largest of the epibranchials; its dorsal end with 

 two articular prominences, the anterior of which con- 

 nects with the first pharyngobranchial, while the poste- 

 rior process connects with the base of the second 

 pharyngobranchial. First pharyngobranchial (suspensory 

 pharyngeal) a curved rod; toothless; articulates ventrally 

 with the first epibranchial and dorsally with the lateral 

 surface of the base of the large ventral process of the 

 posteroventral surface of the parasphenoid. 



posteromedially with the second basibranchial and dor- 

 sally with the second ceratobranchial. Second cerato- 

 branchial the longest of the ceratobranchial elements; 

 articulates dorsally with the second epibranchial. Second 

 epibranchial columnar, with a slight expansion postero- 

 ventrally; articulates dorsally with the second pharyngo- 

 branchial. Second pharyngobranchial roughly L-shaped, 

 with an irregular series of about 25 well-developed and 

 sharp-pointed teeth borne on the slightly longer of its two 

 arms. The toothless arm of the pharyngobranchial ar- 

 ticulates ventrally with the posterodorsal process of the 

 first epibranchial and with the dorsal end of the second 

 epibranchial. The teeth are borne in individual sockets 

 and are replaced by new teeth developing in sockets 

 irregularly scattered between the sockets of the old teeth. 

 The second pharyngobranchial is closely held to the 

 other two toothed pharyngobranchials by fibrous tissue. 



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

 pharyngobranchial elements present. Third basi- 

 branchial the longest of the basibranchials; laterally ex- 

 panded in the anterior two-thirds of its length; articu- 

 lates anteriorly with the second basibranchial, postero- 

 laterally with the third hypobranchials, and posteriorly 

 with the fourth ceratobranchials. Third hypobranchial 

 produced anteroventrally, at which end it articulates by 

 fibrous tissue with the ventral surface of the first basi- 

 branchial; articulates posteromedially with the third 

 basibranchial and posterolaterally with the third cerato- 

 branchial. Third ceratobranchial like the others; ar- 

 ticulates dorsally with the third epibranchial. Third epi- 

 branchial triangular in shape; articulates anterodorsally 

 with the third pharyngobranchial and posterodorsally 

 with the middle of the anterior edge of the fourth epi- 

 branchial. Third pharyngobranchial rounded ventrally, 

 only slightly expanded dorsally; toothed in the same 

 manner as the second pharyngobranchial, but with 

 somewhat fewer teeth on its slightly shorter and thicker 

 tooth-bearing edge. 



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

 branchial elements present. Fourth ceratobranchial like 

 the others; articulates ventrally with the third basi- 

 branchial and dorsally with the fourth epibranchial. 

 Fourth epibranchial a column expanded posteriorly in 

 the middle of its length; articulates dorsally with the 

 fourth pharyngobranchial. Fourth pharyngobranchial 

 the smallest of the toothed pharyngobranchials, but with 

 the widest tooth-bearing surface; teeth like those of the 

 other pharyngobranchials, except slightly smaller. 



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

 pharyngobranchial elements present. Second basi- 

 branchial only slightly longer than the first basi- 

 branchial, but much wider; articulates anteriorly with 

 the first basibranchial, anterolaterally with the first 

 hypobranchials, posterolaterally with the second hypo- 

 branchials, and posteriorly with the third basibranchial. 

 Second hypobranchial the largest of the hypo- 

 branchials; much expanded anterolaterally; articulates 



Fifth arch. — Ceratobranchial (lower pharyngeal) 

 element only. Fifth ceratobranchial a stout shaft; round- 

 ed anteroventrally but tapering to a narrow blunt end 

 posterodorsally; articulates anteroventrally with the 

 base of the fourth ceratobranchial. The rounded anterior 

 region of the fifth ceratobranchial bears teeth like those 

 of the pharyngobranchials, except slightly smaller, but 

 more posteriorly on the dorsal surface of the shaftlike 

 portion the teeth are extremely small. 



249 



