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Show 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 are 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, I. 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. Very 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 ‘ Vergleichenden Anatomie der Wii'belthiere,' 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 anteroposterior compression, so that all its segments, and more especially the epibranchial and ceratobranchial pieces, assume the form of relatively thin cartilaginous 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 are nearly anterior and posterior respectively. As usual in other fishes, the gill-rakers of the first four branchial arches form two rows in relation with each arch; and, * " Notice of the Spoonbill Sturgeon or Paddle Pish of the Ohio (Polyodon feuille of LacopMe)," Silliman's Amer. Journ. of Science, vol. xii. (1827) p. 203 t Ibid. p. 204. J ‘ Comparative Anatomy,' vol. i. p. 482. § Brit, Mus. Cat. of Fishes, vol. viii. (1870) 346, |