edge of the ceratohyal; third branchiostegal with a slight 

 concavity on the posterior edge of the ceratohyal; fourth 

 branchiostegal with the cartilage between the epihyal 

 and ceratohyal; fifth and sixth branchiostegals with the 

 lateral surface of the epihyal. 



Branchial Arches. —The elements are cartilage filled 

 at their edges of articulation with the other elements of 

 the series, and the articulations are usually through car- 

 tilage and fibrous tissue. The branchial arches are com- 

 posed of three basibranchials, three pairs of hypobran- 

 chials, five pairs of ceratobranchials, four pairs of epi- 

 branchials, and three pairs of pharyngobranchials. Four 

 gills are present, with a large slit between the fourth arch 

 and the lower pharyngeal. The gills are enormously 

 enlarged and only about the lower third of their lengths 

 are supported by the branchial arches; these were de- 

 scribed long ago by Alessandrini (1839) and most recent- 

 ly, in great detail, by Adeney and Hughes (1977). The 

 anterior edges of the upper two-thirds of their lengths are 

 held by fibrous tissue to the ventral surfaces of the pos- 

 terior end of the cranium and the first several vertebrae. 



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

 chial elements present. First basibranchial laterally 

 compressed anteriorly; about the same length as the 

 third basibranchial and somewhat shorter than the sec- 

 ond basibranchial; with a ventral keel medially on its 

 ventral surface; displaced forward so that it articulates 

 posteriorly with the second basibranchial and postero- 

 laterally with the first hypobranchials. First hypobran- 

 chial the longest of the hypobranchial elements, which 

 decrease in length posteriorly in the series; articulates 

 dorsally with the first ceratobranchial, ventrally with the 

 second basibranchials and anteroventrally with the first 

 basibranchial. First ceratobranchial slightly longer and 

 deeper than the other ceratobranchials, which are about 

 equal in length but which decrease in depth posteriorly 

 in the series; articulates ventrally with the first hypo- 

 branchial and dorsally with the first epibranchial. First 

 epibranchial a slender rod articulating dorsally with the 

 fibrous tissue sheet between the dorsal edges of the 

 metapterygoid and hyomandibular and the ventral edge 

 of the parasphenoid. 



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

 ryngobranchial elements present. Second basibranchial 

 the longest and widest of the basibranchial elements; 

 with a ventral keel medially on its ventral surface; artic- 

 ulates anteriorly with the first basibranchial, anterolat- 

 erally with the first hypobranchials, posterolaterally with 

 the second hypobranchials, and posteriorly with the 

 third basibranchial. Second hypobranchial only slightly 

 shorter and narrower than the first hypobranchial; artic- 

 ulates ventrally with the anterolateral edge of the third 

 basibranchial and posterolateral edge of the second basi- 

 branchial, while dorsally it articulates with the second 

 ceratobranchial. Second ceratobranchial articulated dor- 

 sally with the second epibranchial. Second epibranchial 

 a very narrow rod articulating dorsally with the middle of 



the toothless edge of the second pharyngobranchial. Sec- 

 ond pharyngobranchial a rounded plate bearing about 

 four long teeth set in sockets along its ventral edge. The 

 bases of the teeth are expanded and articulate with their 

 sockets by fibrous tissue. 



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

 ryngobranchial elements present. Third basibranchial 

 rectangular in shape; its ventral surface without a keel; 

 articulates anteriorly with the second basibranchial, 

 anterolaterally with the second hypobranchials, and pos- 

 terolaterally with the dorsal ends of the third hypobran- 

 chials and the ventral ends of the third ceratobran- 

 chials, while posteriorly it supports the cartilaginous 

 area to which are attached the fourth and fifth cerato- 

 branchials. Third hypobranchial a short column of bone, 

 almost vertical in position and sunken below the general 

 connective tissue that lies between the rest of the basi- 

 branchials and hypobranchials; articulates dorsally with 

 the ventral surface of the cartilage between the postero- 

 lateral end of the third basibranchial and the ventral end 

 of the third ceratobranchial, while its ventral end con- 

 nects by a band of fibrous tissue with the ventral keel of 

 the second basibranchial and, continuing anteriorly, 

 with the ventral keel of the first basibranchial. Third 

 ceratobranchial articulated dorsally with the third epi- 

 branchial. Third epibranchial a stout rod; articulates 

 dorsally with the third pharyngobranchial. Third pha- 

 ryngobranchial the largest of the pharyngobranchial ele- 

 ments; bears about six long, sharp-pointed teeth whose 

 expanded bases articulate by fibrous tissue with their 

 sockets on the ventral edge of the pharyngobranchial; ar- 

 ticulates ventrally with the third epibranchial and ante- 

 riorly and posteriorly with, respectively, the second and 

 fourth pharyngobranchials. 



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

 chial elements present. Fourth ceratobranchial articu- 

 lated ventrally with the third basibranchial and dorsally 

 with the fourth epibranchial. Fourth epibranchial a large 

 flattened plate, wider ventrally than dorsally; articu- 

 lates ventrally with the dorsal ends of the fourth and fifth 

 ceratobranchials and dorsally with the fourth pharyngo- 

 branchial. Fourth pharyngobranchial similar to the sec- 

 ond pharyngobranchial; bearing four teeth in a single 



Fifth arch. — Ceratobranchial (lower pharyngeal) 

 element only. Fifth ceratobranchial like the others in the 

 series, but not as deep; toothless; articulates ventrally 

 with the base of the fourth ceratobranchial. 



PAIRED FIN GIRDLES. 



Pectoral Fin. 



Supracleithrum. —Long and flat, somewhat 

 expanded ventrally where it overlies the cleithrum; in 

 position more or less parallel to the posterior end of the 

 skull; articulates by fibrous tissue posteromedially with 



373 



