24 MR. A. D. IMMS ON THE GILL^RAKERS [May 3, 



also for his kindness in placing ample material at my disposal and 

 for valued assistance rendered to me in various ways. 



Anyone who has made even a cursory examination of the gills 

 of this fish cannot fail to have been struck with the appearance 

 of the regular comb-like organs which the rows of gill-rakers form 

 on each face of the branchial arches. Although they are familiar 

 to most zoologists and are characteristic of Polyodon^ no one, so 

 far as I am aware, has devoted to them more than passing notice. 

 From among the early accounts of this fish it will serve the present 

 purpose sufficiently if reference be made to a single source only. 

 It is in a letter by Dr. S, P. Hildreth to the editor of the ' American 

 Journal of Science ' that the following mention is made of the gill- 

 rakers. He remarks: " The jaws ai-e without teeth ; but the fauces 

 are lined with several tissues of the most beautiful network, 

 evidently for the purpose of collecting its food from the water, by 

 straining, or passing it through the ciliary membranes, in the 

 same manner as practised by the spermaceti whale." * Another 

 observer, T, W. Clemens, says that Polyodon " had five pairs of 

 gills which were double. Each of these duplicatures were thickly 

 set with teeth, of about the diameter and consistence of best 

 Russian bristles, and one and a fourth inches long."t He 

 mentions that the particular fish he examined measured 4 ft. 8 in. 

 in length. 



Yery little further information is to be gleaned from the works 

 of any of the later writers. Reference is made to their occurrence 

 in this fish by Owen J ; and Gunther§ remarks that each branchial 

 arch has a double series of very long setiform gill-rakers, and that 

 the two series are separated by a broad membrane. No adequate 

 figures of them appear to have been published by any author. 

 A representation of a branchial arch of Polyodon, which also 

 shows the very characteristic disposition of the gill-rakers, is given 

 by Prof, Wiedersheim in the 2nd edition of his ' Yergleichenden 

 Anatomie der Wirbelthiei-e,' but has been omitted in the later 

 edition. 



Before describing the gill-rakers of Polyodon, it will be necessary 

 to refer to certain peculiar features in connection with the 

 branchial arches. Each arch has undergone a remarkable antero- 

 posterior compression, so that all its segments, and more especially 

 the epibranchial and ceratobranchial pieces, assume the form of 

 relatively thin cai'tilaginous plates, The plates are so obliquely 

 disposed that the proper anterior and posterior surfaces look 

 outward and inward respectively, while the concave inner and the 

 convex outer margins a.i'e nearly anterior and posterior respec- 

 tively. As usual in other fishes, the gill-rakers of the first four 

 branchial arches foi'm two rows in relation with each arch ; and, 



* " Notice of the Spoonbill Sturgeon or Paddle Fish of the Ohio {Polyodon feuille 

 of LacepMe)," Silliman's Amer. Jouvn. of Science, vol. xii. (1827) p. 203, 

 t Ibid. p. 204. 



X ' Comparative Anatomy,' vol. i. p. 482. 

 § Brit, Mus. Cat. of Fishes, vol. viii. (1870) p. 346, 



