1904,] OP THE SPOONBILL STURGEOJN. " 23 



occur as a single or double row of protuberances disposed along 

 the concave inner or pharyngeal margins of the branchial arches. 

 They may be modified, however, so as to become tooth-like, seti- 

 form, or even filamentous. In some fishes they are completely 

 absent. Among the Elasmobranchs these structures are found in 

 a number of species, and, as a typical example, they are well seen 

 in Acanthias vulgaris. In this fish they occur as lanceolate 

 projections which are developed principally along the anterior 

 edges of the pharyngeal margins of the arches. In the " Basking 

 Shark," Ceiorhinus {Selache) inaxmius, they are found as a series 

 of greatly elongated, coarse, seta-like structures which are disposed 

 along the inner margins of the branchial arches upon both the 

 anterior and posterior edges. A somewhat similar apparatus is 

 described as being present in the South African " Whale Shark," 

 Rhinodon typicus. Among the Holocephali the gill-rakers are 

 small in size and are not conspicuous ; they are seen in Ghi'moera as 

 rows of small tubercles. By far the greater number of the 

 Teleostomes possess gill-rakers, and it is in this subclass that they 

 exhibit their widest range of variation. A. contribution towards 

 a systematic study of them in the freshwater members of the 

 Teleostomi has been made quite recently by Zander*. He has 

 examined a considei-able niimber of species, and finds that in 

 carnivorous types like Esox and Lucioperca the gill-rakers are in 

 the form of teeth. In numerous other genera they form a sieve- 

 like filtering-apparatus {" Siebfortsatze"), which is developed in 

 some cases upon both edges of the branchial arches, as in Percay 

 Acerina, Lota, and the Cyjirinidae ; or in others only upon the 

 anterior edges, as in Gliipea alosa, Coregomis fera, C. alhula, and 

 Osmerus. The relative fineness of the "Siebfortsatze " is correlated 

 with the nature of the food of the species where they occur, and 

 attains its extreme development in this respect amongst those 

 fishes which subsist upon plankton. Popta t also has studied the 

 gill-rakers in the Teleostomi, and has given brief descriptions of 

 their structure and disposition in numerous species. As the 

 result of his studies, he finds that they are specifically peculiar or 

 diagnostic in all the forms which he examined, and interprets 

 their arrangement and development in relation to the form of the 

 mouth and the nature of the food. All three genera of the 

 Dipnoi possess gill- rakers. They are largest and stoutest in 

 JSfeoceradotus. In Protopterus and Lepidosiren they are present 

 in the condition of minute, slender, pointed projections, and in 

 the specimens which I examined they are slightly more delicate 

 in the first mentioned of the last two genera. 



The object of the present article is to call attention to some 

 interesting features with regard to the structure and function of 

 gill-rakers in the case of the " Spoonbill " or " Paddle Fish," 

 Polyodon spathula. I am indebted to Prof. T. W. Bridge, F.R.S,, 

 for suggesting to me that I should examine these organs, and 



* Zeitsch. fur wiss. Zool. Bd. Ixxv. 1903, pp. 233-258. 



t Ann. Sci. Nat., Zool. t. xii. 190O, pp. 139-216. 



