32 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 light angle with the gill-ray, and 

 effectually barred the egress through the gills of anything except 

 water taken in thi-ough 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 Miiller, 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 measiu^es 88"4 cm. (2 ft. 10 in.) 

 long, I find that there are unmistakable teeth arranged on the 

 jaws, as Miiller states ; those in the upper jaw are worn down a 

 little more than those in the lower. In another fish measuring 

 13Q-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 stirring 

 up with its spatxilate 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 it ; 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 i, 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, Avhich consists 

 chiefly of entomostracans, water-worms, aquatic plants, leeches, 

 beetles, and insect larvfe." 



In the hope of being able to furnish some additional obser- 

 vations, 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 Myxiuoiden,' p. 150. 

 1 Z,oc. cit. p. 204. 



§ " Cat. of the Fishes of New Yort," Bull. 60 of the New York StateMusenm, 1903, 

 p. 62. 



