1904.] OF THK SPOONBILL STURGEON. 25 



again following the general rule, they are attached to the anterior 

 and posterior surfaces, close to the concave inner margin, in the 

 form of an anterior and a posterior series, but from the obliquity 

 of the surfaces of the arch the two series appear as if disposed 

 along the outer and inner faces of an arch. They will be referred 

 to in future, however, as the anterior or the posterior series, as 

 the case may be. 



If the gill- rakers are surveyed from the first branchial arch 

 backwards to the fourth arch, they are seen to become progres- 

 sively shorter in length, and, furthermore, those of them that 

 are situated along the outer or antei-ior aspect of an arch 

 are somewhat longer than those carried along the inner or 

 posterior aspect of the same arch (PI. II. fig. 2, i.g.r. and o.g.r.). 

 Hence the series of the longest gill-rakei's is carried on the outer 

 aspect of the first branchial arch, while the row borne on the inner 

 aspect of the fourth branchial arch is composed of the shortest 

 gill-rakers. It is also worthy of note that the longest gill-rakers 

 in either an anterior or a posterior series of a branchial arch are 

 those situated nearest to the junction of a ceratobranchial with 

 an epibranchial element (fig. 1, r.), the gill-r-akers gradually 

 increasing in length from the dorsal and ventral extremities of 

 an arch until the centi'e of the concavity is reached, where they 

 attain their maximum. The fifth branchial arch carries gill-rakers 

 along its anterior surface only, and they are slightly longer than 

 those disposed along the posterior aspect of the preceding arch. 

 The fifth gill-arch is itself much reduced, since it retains only its 

 ceratobranchial element, and, in correspondence with this, gill- 

 rakers are not developed on the opposing face of the preceding 

 arch in relation to its epibranchial cartilage, but only with the 

 ceratobranchial *. 



Owing to the extraordinary compression of the plate-like 

 branchial archest, combined with the attachment of the gill- rakers 

 to their concave inner margins, the greater portion of each arch 

 practically forms a stout cartilaginous septum, which separates 

 the anterior from the posterior series of gill- rakers in relation 

 with each arch, much in the same way that an inter-branchial 

 septum would separate the double series of gill-filaments on the 

 opposite or outer margin of a branchial arch (figs. 1 & 2). The 

 necessity for this curious modification is by no means obvious. 

 It would seem that, were the septum absent, such delicate 

 and fragile organs as the gill-rakers would be very liable 

 to get dislocated or clogged together, and perhaps damaged, 

 through one series of gill-rakers rubbing against the other. 

 As it is, each row is kept in a beautifully regular order, and 

 not a single gill-raker will be noticed to be disarranged from 

 its proper position, and all of them when not in use are closely 

 applied to the surface of the septum. The function of this septum 



* A full account of the skeleton of the visceral arches of Folyodoyi is given b.y 

 Prof. T. W. Bridge in the Phil. Trans. 1878,vol. 169 ; vide pp. 702-712 & pi. 57. figs. 8 & 9. 

 t Lacepede (1798) speaks of the branchial arches as cartilage-plates. 



