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Show 33 MR. A. D. IMMS ON THE GILL-RAKERS [May 3, Cornish has stated, in an account of a supposed Basking Shark, that in front of each gill a slight comb-like apparatus extended the whole length of the ray. As the mouth was opened, the comb automatically fell back to a right angle with the gill-ray, and effectually barred the egress through the gills of anything except water taken in through the mouth*. Although this apparatus has received attention from numerous zoologists, no one, so far as I am aware, has offered any suggestion as to the means by which the gill-rakers are brought to interlock with one another when they are in use. The occurrence of minute teeth in Polyodon is a well-known feature. According to Johannes Milller, there are found in young specimens (a foot long) two rows of small teeth in the upper jaw and one row in the lower jaw. Similar teeth are found on the two anterior branchial arches where they join the floor of the mouth, and upon their opposite extremities where they join the palate. He mentions that examples over 3 feet long are edentulous f . In a specimen in the Zoological Museum of the University of Birmingham which measures 88'4 cm. (2 ft. 10 in.) long, I find that there are unmistakable teeth arranged on the jaws, as Muller states; those in the upper jaw are worn down a little more than those in the lower. In another fish measuring ] 39'1 cm. (4 ft. 5| in.) in length, I have been unable to detect any trace of teeth. The nature of the food of Polyodon is correlated with the vestigial character of the teeth. The fish is described as stining up with its spatulate snout the mud at the bottom of the waters of the " bayous and lowland" streams which it frequents, and feeding upon the microscopical organisms contained in i t ; but the evidence which supports such a statement appears to be rarely quoted, and it leads one to believe that it is not so definite as one would wish. An early writer, already referred to, namely I. W. Clemens X, remarked that the Polyodon which he dissected " had no food in its intestines-all that was observable was a small quantity of substance resembling chyle, but of the consistence of honey." T. H. Bean §, quoting Prof. S. A. Forbes, says that " the long snout is useful in procuring its food, which consists chiefly of entomostracans, water-worms, aquatic plants, leeches, beetles, and insect larvse." In the hope of being able to furnish some additional observations, I made a careful microscopical examination of the contents of the whole course of the alimentary canal in two specimens of Polyodon. In both cases the food appeared to have been much acted upon by the digestive secretion and very little * ‘ Zoologist,' 1870, p. 2253. t ‘ Anatomie der Myxinoiden/ p. 150. I Loc. cit. p. 204. § " Cat. of the Fishes of New York," Bull. 60 of the New York State Museum 1903 p. 62. ' |