1904.1 OF THE SPOOXBILL STURGEOX. 33 



could be made out with regard to its nature. The specimens 

 liad been in spirit for a long time, which greatly increased 

 the difficulty of identification. I was able, however, to recognise 

 among it i-emains of parts of the exoskeleton of insect larvae 

 together with portions of the spiral thickenings of the tracheae, a 

 few small Oligocha^te worms, fragments of plant-remains, and 

 some earthy matter. 



There is every reason to believe that the gill-rakers of Polyodon 

 are similar in function to those of Cetorhinus. On account 

 of their fineness, and the closeness with which they ai'e packed 

 too-ether, they would form an even more effective straining- 

 apparatus than they do in the case of the lattei- fish. 



In the absence of direct observations on the living fish, as to the 

 precise method by which the gill-rakers constitute an efficient 

 filtering-mechanism, i-ecoui'se must be had to anatomical evidence. 

 If the gill-i-akei'S act as a filter, it is clear that the anteiior series 

 of rakei'S of one branchial ai'ch must be inclined forwards so as 

 to meet the postei-ior series of the pi'eceding arch, which have 

 become inclined backwaixls for the purpose, so that the two series 

 of giU-rakei'S interlock or intei'digitate across the cleft. Under 

 ordinary circumstances, however, the gill-iukers are closely 

 applied to the anterior or posterior surfaces of the flattened 

 In'anchial arches to which they belong, and they do not in 

 the least incline across a cleft, or tend to meet those of an 

 adjacent arch on the opposite side of the cleft. In all pi'obal)ility 

 the necessary movements of the gill-i-akers are brought about by 

 means of the conti'actions of the muscle-fibres, repi*esented in PI. II. 

 fio-. 5 which extend downwards and inwards from the outside of 

 a oill-raker and are attached to the cai'tilage of the branchial 

 arch. By the contraction of these miiscle-fibres the gill-rakei-s 

 would be pulled outwards so as to form an angle of about 60° with 

 the septum. When the fibres are relaxed, the gill-rakers, on 

 account of the pull that is exerted upon them by the sti'etched 

 elastic fibi'es, w^ould spiing back of their own accoi'd to close against 

 the septum, and so take up the position they occupy when not in use. 

 The anterior row carried by the first gill-arch is composed, as 

 already mentioned, of the lai-gest individual gill-rakers. The 

 reason foi- this appears to be that they have to biidge over the 

 interval between that arch and the hyoid, since the latter is devoid 

 of gill-rakers. 



In concluding these few notes it may be worth while to mention 

 that the gill-rakers of the other surviving Chondrostean Ganoids 

 difl^er very greatly from those of Polyodoiij, although all these 

 Fishes share the common chai'acter of having a greatly i-educed 

 dentition. In Psephurus they ai-e comparatively shoi-t and are 

 moderate in number. In Scaphirhynchus they are small fan- 

 shaped sti'uctures each of which terminates in thi-ee or four points. 

 In Acipenser the gill-i;akers are small, flattened, and pointed organs 

 which (lifter somewhat in relative size among the various species. 



Proc. Zool. See— 1904, Vol. II. No. III. 3 



