1904.] OF THE SPOONBILL STURGEON. 29 



contains large numbers of coarse dentine-tubes which arise from 

 the walls of the canals and run outwards to the periphery, where 

 they form a layer of hard dentine. Turner regards these gill- 

 rakers as being composed of vaso-dentine, but Tomes in referring 

 to them adds " (? osteo-dentine)." * 



It is worthy of note that in fishes of the genus Chcetodon and 

 their allies the maxillary teeth appear to have been modified along 

 the same hnes as those by which the setiform type of gill-raker 

 has been produced. As their name implies, the teeth of these 

 fishes are bristle-like ; they resemble the hairs of a fine brush in 

 being flexible and elastic, and they are composed of a yellowish, 

 shining, semi-transparent tissue t. 



I would suggest that possibly the gill-rakers of Polyodon are 

 morphologically the much modified descendants of exoskeletal 

 structures which have migrated along with the ectoderm on to the 

 branchial arches. The fact that the mucous membrane covering 

 the branchial arches is regarded as being endodermal in origin, 

 offers considerable difficulty to any idea that such structures could 

 have developed there independently and in situ, unless they have 

 arisen in the underlying mesoblast. Klaatsch, however, from a 

 study of the placoid scales in Mustelus and some other Elasmo- 

 branchs, has arrived at the conclusion that their scleroblasts are 

 ectodermal in origin and are derived from the same layer as that 

 which gives rise to the enamel. This layer, which is at first homo- 

 geneous, becomes divided into a portion which has been usually 

 considered to be of mesodermal origin, while the rest remains in 

 connection with the ectoderm J. Hence he considers that a 

 " dermal " exoskeleton is not mesodermal in its ultimate origin. It 

 is worthy of note that, with regard to the pharyngeal teeth of 

 many fishes, several writers are inclined to believe that their 

 presence is due to a migration of the ectoderm into the cavity of 

 the pharynx. For this reason, and on account of the difficulty of 

 reconciling them with the presence of anything except ectoderm, 

 I would suggest the possibility that the skeletal tissue of the gill- 

 rakers of Polyodon has arisen fiom portions of the epiblast 

 forming the outer portions of the gill-clefts, which have migrated 

 on to the inner or pharyngeal margins of the branchial arches. At 

 all events, if any migration of epiblast has taken place, the latter 

 route seems at least as feasible as a backward migration from the 

 stomodaeum. 



In Getorhinus the gill-rakers retain many structural features in 

 common with the teeth of the animal, but in Polyodon they 

 appear to have undergone a more special modification along lines 

 of their own. The structure of the teeth in the young Polyodon 

 has been described by Zograff'§, but, after a comparison of the 



* ' Dental Anatomy,' p. 220. 



f Vide Owen, ' Odontography,' pp. 8 & 105, pi. i. fig. 2. 

 X Morpli. Jahrb. xxi. 1894, pp. 153-2<W. 



§ " Ueber die Zahne der Knorpel-Ganoiden," Biol. Centralbl., Bd. vii. 1887-88, 

 p. 181. Ann. Sci. Nat., Zool. 8 ser. t. i. p. 203, pi. 4. figs. 3, 4, & 6. 



