46 



THE ELASMOBRANCH FISHES 



downward, the pharyngobranchial and ceratobranehial lying almost parallel 

 to each other. All the branchial arches, except the last, are similar to the first. 

 They diifer from it, however, in that their hypobranchial segments are better 

 developed. In the last arch a functional hypobranchial is lacking and the 

 pharyngobranchial has fused dorsall}^ with that of the sixth arch. 



A B 



Fig. 50a. The median branchial cartilages, Heptanchus maculatus, dorsal view. (H. M. 

 Gilkej, del.) 



Fig. 50b. Area of rudimentary arches, Heptanchus maculatus, ventral view. 



ctr.", ninth arch; hb.', second basibranchial ; hh., basihyoid; ch., ceratobranehial; ch., 

 ceratohyoid; hh., hypobranchial; mp., median piece; r., rudimentary rays; x., part of 

 eighth arch. 



In the midventral line of the branchial basket is a series of unpaired pieces 

 which unite the arches of opposite sides. The first of these is the basihyoid pre- 

 viously mentioned (hh., fig. 50a). A first basibranchial piece is lacking, 

 but basibranchials are present on the second to the fourth arches following 

 (hh-''^) . A large median piece posteriorly {mp.) serves for the attachment of 

 the fifth and sixth hypobranchials and the seventh ceratobranehial (cbJ). 



All these arches, except the last, have cartilaginous branchial raj's on their 

 epi- and ceratobranehial segments. These branchial rays, though simpler than 

 those on the hyoid arch {h.r., figs. 48 and 49), serve the same function of sup- 

 porting the gill septa. 



The visceral arches are further provided with superficially placed pieces, 

 the extravisceral cartilages. Those accompanying the first or mandibular arch 



