by fibrous tissue posteriorly with the first basibranchial 

 and anteriorly with the reduced urohyal, while the dorso- 

 medial edge of the ventral hypohyal also articulates by 

 fibrous tissue with the urohyal. 



Ceratohyal. — A wide flat plate; shortened antero- 

 posteriorly and expanded dorsoventrally, particularly 

 posteriorly; cartilage filled along all of its edges except 

 for the indented regions dorsally and ventrally; ar- 

 ticulates through cartilage anteriorly with the dorsal and 

 ventral hypohyals, and posteriorly with the epihyal. The 

 first two branchiostegal rays articulate by fibrous tissue 

 with slight concavities on the anterior half of the ventral 

 edge of the ceratohyal. The next three branchiostegal 

 rays articulate by fibrous tissue with the posteroventral 

 edge of the ceratohyal, and the last ray with the ventral 

 region of the lateral face of the epihyal and, to a lesser ex- 

 tent, the lateral face of the extreme posterior end of the 

 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 column; cartilage filled at both 

 ends; articulates by fibrous tissue ventrally with the epi- 

 hyal and dorsally with the concavity on the ventral edge 

 of the metapter>'goid immediately behind the postero- 

 dorsal end of the symplectic. 



Branchiostegal rays. — Six in number; the last ray 

 slightly longer than the others; articulates by fibrous tis- 

 sue to the ceratohyal and, to a lesser extent, the epihyal, 

 as explained under those bones. 



Urohyal. —Reduced to a somewhat irregularly 

 curved rod without a ventral keel; articulates by fibrous 

 tissue anterodorsally with the anteromedial edge of the 

 dorsal hypohyal and the dorsomedial edge of the ventral 

 hypohyal; articulates by fibrous tissue posterodorsally 

 with the ventral surface of the first basibranchial. 



branchial short, only 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 a square block of about the same size as the 

 second hypobranchial; articulates ventrally with the 

 lateral surface of the second basibranchial and dorsally 

 with the first ceratobranchial. First ceratobranchial a 

 sturdy rod; about equal in length to the other cerato- 

 branchial elements; somewhat greater in depth ven- 

 trally than dorsally, this increased depth at the ventral 

 end increasing from the first to the last ceratobranchial, 

 the last being widest in the middle region; 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 

 sturdy column; articulates anterodorsally with the base 

 of the first or suspensory pharyngobranchial. First 

 pharyngobranchial a toothless flattened plate; ar- 

 ticulates dorsally by fibrous tissue to the undersurface of 

 the extreme anterior end of the prootic suborbital 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 largest of the three elements in the series; 

 articulates anteriorly with the first basibranchial, 

 laterally with the first hypobranchial, and posteriorly 

 with the third basibranchial. Second hypobranchial ar- 

 ticulated ventrally with area of articulation between the 

 second and third basibranchials and dorsally with the 

 second ceratobranchial. Second ceratobranchial ar- 

 ticulated dorsally with the second epibranchial. Second 

 epibranchial a wide heavy column; articulates dorsally 

 with the base of the second pharyngobranchial. Second 

 pharyngobranchial the largest of the four pharyngo- 

 branchial elements and the first of the tooth bearing 

 elements. The teeth are very small and difficult to see 

 and in life may largely be buried in the pharyngeal tis- 

 sues. The 15 to 20 minute teeth on the second pharyngo- 

 branchial are much shorter than those on the third and 

 fourth pharyngobranchials, and are grouped together in 

 the central region of the ventral face of the element. 



Branchial Arches. — AH 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 four pairs of pharyngo- 

 branchials. Four gills are present, with a small slit 

 between the fourth arch and the lower pharyngeal. 



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

 pharyngobranchial elements present. First basi- 



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

 pharyngobranchial elements present. Third 

 basibranchial a short column, the shortest of the three 

 basibranchials; articulates anteriorly with the second 

 basibranchial, anterolaterally with the second 

 hypobranchials, posterolaterally with the third hypo- 

 branchials and third ceratobranchials, and posteriorly to 

 the region between the fourth certobranchials. Third 

 hypobranchial a long slender rod, directed anteroventral- 

 ly and with only its extreme posterodorsal end at the 

 level of the other hypobranchial elements; articulates 

 posterolaterally with the third ceratobranchial and 



