SKELETON OF EBGAXECUS ARGENTEFS. 17 



5. The Beanchial Arches. (Plate V. fig. 17.) 



These are also quite typical in their general arrangements, consisting of five pairs 

 of arches united ventrally by a median longitudinal series of azygos pieces or copulcB. 

 As regards the details of segmentation, however, there are a good many variations 

 from the usual state of things. 



The first arch is the longest, and consists of four segments ; the ventralmost of these 

 IS a comparatively long hypo-hrancMal {h.hr^); next comes a long cerato-hranchial 

 {c.hr ') ; then a short epihrancMal {e.br ') like the two former, of a rod-like form, and 

 grooved on its outer side for the branchial vessels ; then a wider and more flattened 

 pharyngo-hranchial {ph.hr ^): and finally a slender rod of cartilage {pa.br ^) with an 

 extremely thin ossific crust, articulated to the dorsal end of the pharyngo-branchial, 

 and lying in the natural position of the parts, almost vertically against the subcranial 

 crest, close behind the postorbital pillar (tig. &, pa.br^). I can find no mention of any 

 segment corresponding to this in other fishes, and propose to name it the parabranchial. 

 A further peculiarity is the presence of a small distinct nodule of cartilage (ic) 

 articulated to the dorsal end of the cerato-branchial, internal to the epibranchial. 



The second arch closely resembles the first as to its hypo- {li.br') and cerato- 

 branchial {c.br'^) segments. The epibranchial {e.br'') is bent upon itself almost at a 

 right angle ; the pharyngo-branchial {ph.br ^) is much broader and flatter than in the 

 first arch, and articulates with the epihyal, not by its end, but by a small cartilage- 

 covered tubercle on one side ; with its narrow anterior end is articulated a para- 

 branchial {pa.br ^) slightly larger than that of the first arch, and quite unossified. 



In the third arch the hypobranchial {h.br^) presents a modification very usual in 

 Teleosts : it is produced into a downwardly directed hook-like process (shown in the 

 figure by a dotted outline), which, with its fellow, helps to bound the canal for the 

 ventral aorta. There is a large synchondrosis between this segment and the cerato- 

 branchial {c.br 2), which latter calls for no special remark. The epibranchial {ejJ.br^} 

 is a forked bone, the anterior branch of the fork, or processus articularis, evidently 

 answering to the forwardly directed limb of the second epibranchial, while the 

 posterior branch, or processus muscularis, is represented in the second arch only by 

 a slightly projecting knob at the bend of the epibranchial. The pharyngo-branchial 

 ( ph.br"^) is similar to that of the second arch, to the outer surface of which it is 

 closely applied : iu the figure it is unnaturally separated from the epihyal, by the 

 parts being spread out in one plane for convenience of sketching. There is no para- 

 branchial in this or in either of the succeeding arches. 



In the fourth arch the hypobranchial is absent : as in the preceding arches a small 

 nodule of cartilage {x ^) is articulated to the upper end of the cerato-branchial {c.br *). 

 The epibranchial {ep.br^) is similar to that of the preceding arch, to which it is closely 

 applied. The pharyngo-branchial {ph.br^) is a small flat bone articulated by a step- 

 like process with the third pharyngo-branchial. 



VOL. XII. — PART I. No. 3. — February, 1886. d 



