34 U.S. NATIONAL MUSEUM BULLETIN 2 84 



side of the urohyal overlaps and attaches to the ventrolateral surface 

 of its respective ventral basihyal. The saddle-Uke depression thus 

 formed between the projections on either side of the urohyal strongly 

 interlocks the ventral hypohyals. 



Each epihyal and ceratohyal join by an interdigitating joint. Each 

 ceratohyal bears two deep notches ventroposteriorly in each of which 

 a branchiostegal (first and second branchiostegals) is attached. The 

 third and fourth branchiostegals attach to the lateral surface of the 

 posterior end of the ceratohyal; the fifth and sixth branchiostegals 

 attach to the lateral surface in the area of the interdigitating joint 

 between the ceratohyal and epihyal. The anteroventral surface of the 

 interhyal connects to the dorsoposterior surface of the epihyal. The 

 posterior end of the interopercle attaches to the epihyal just anterior 

 to the attachment of the interhyal. Strands of connective tissue ex- 

 tend from the posterior surface of the urohyal to the ventral junction 

 of the two cleithra. 



Branchial arches (fig. 9). — The branchial arches are suspended 

 from the ventral side of the cranium. The first arch is connected 

 ligamentously to the cranium by the first epibranchial. The epi- 

 branchials of the second, third, and fourth arches of each side are 

 attached on their respective side to a single tooth bearing upper 

 pharyngeal bone (probably representing the second through fourth 

 pharyngobranchials) , which is in turn suspended from the cranium. 

 The fifth arch is represented by a single tooth bearing lower pharyngeal 

 bone (fifth ceratobranchial) . A broad ligament attaches the antero- 

 dorsal end of the lower pharyngeal to the dorsoposterior surface of 

 the fourth ceratobranchial. The first and second hypobranchials of 

 each side attach between the first and second and the second and third 

 basibranchials, respectively. The third hypobranchial of each side is 

 directed anteriorly and attached by a ligament from its anterior end 

 to the flattened ventral surface of its respective second hypobranchial. 

 At its posterolateral end, each third hypobranchial is attached to the 

 distal end of its respective third ceratobranchial. A flattened plate of 

 cartilage is situated between, and attached ligamentously to, the 

 posteromesial ends of the third hypobranchials of both sides. This 

 same cartilage also attaches to the distal ends of the fourth cerato- 

 branchial and lower pharyngeal bone of each side. Each of the first 

 four arches bears a double row of short, slender gill-rakers (not illus- 

 trated) anteriorly and a double row of long, slender, bony gill filament 

 supports posteriorly (not illustrated). The fifth arches (lower pharyn- 

 geals) each bear a row of gill-rakers anteriorly, but no gill filament 

 supports. 



Outline of the laterosensory system. — The anteriormost 

 opening of the laterosensory system on each side is through the ventral 



