The Structure of Dinichthys 



145 



Antero'supra'gnathal (pre'inaxilla) and the postero'supra^gnathal (maxilla) are the 

 two elements which together with the infero-gnathal (mandible) form the real chewing 

 apparatus in the Arthrodira. In many of them were developed tooth'Hke tubercles, placed 

 along the edges. The character and structure of these have been studied by many 

 authors. The majority have come to the conclusion that these tubercles have nothing to 

 do with teeth, but are formed of real bone substance. No traces of dentine or enamel are 

 present. Here follow the opinions of various investigators concerning this matter in the 

 Arthrodira. 



Miller in his British Association report (1850, pp. 92-93) says "The teeth of Coc^ 

 costeus . . . were formed of true bone." And Pander in his great work on the Placoderms 

 (1857, PP- 68 and 87) writes thus, "Die kleinen Zahnchen scheinen mit dem Knochen innig 

 verbunden und nur knocherne Fortsatge desselben 2,u sein." While Claypole (1893.1, p. 

 608) notes that "This dense tissue of the mandible in no respect resembles true tooth struc- 

 ture. There is no trace of dentine or osteo-dentine, still less of enamel.'' (See also Clay 

 pole 1893.3, pp. 189-190.) 



Dean (1901.1, p. Ill) says "The study of the minute structures of the jaw elements in 

 the Arthrodira shows that they correspond in all parts to the typical plates of the cranial 

 roof." And Jaekel (1919, pp. 77 and 81) affirms that "Die Zacken [Zaihne] . . . teile des 

 Gan2;en bleiben und sich nicht zu besonderen histologischen Einheiten von dem Knochen- 

 gewebe absondern." (See also Jaekel, 1902, p. 107; 1911, p. 44; 1926, pt. Ill, Heft 3, p. 

 355). Likewise Goodrich (1909, p. 123) declares that "The teeth, indeed, appear to have 

 always been continuous with the supporting bone, and possibly are merely tooth-like 

 processes." 



Hussakof (1906, p. 123) concludes that "The teeth are composed of osseous tissue 

 which is more compact than, but not different from, that in the rest of the dermal element. 

 No dentine is present." While Obrutschew (1927, p. 680) states that "Sections have 

 absolutely confirmed that we are here dealing not with the real teeth from dentine, mor- 

 phologically separated from the jaw, but only with the teeth-like processes of the 

 jawplate." (Orig. in Russian). And last of all Stetson (1930, p. 37) says that "There is no 

 trace of dentine or enamel in the jaws of Dinichthys.''' 



But not all authors are'of the same opinion. Agassiz, in "Poissons Fossiles du Vieux 

 Ores Rouge etc." (1844, pp. 24-26; Tab. B, fig. 2) described the minute structure of a 

 Coccosteus tooth. According to him, this tooth has no pulp-cavity. Its center was occu- 

 pied by a network of medullary canals, from which radiated fine dental canals. Nearly 

 the same description was given by M'Coy in 1854 who stated (p. 602) that "The teeth 

 have the middle occupied by a coarse network of irregular, tortuous, medullary canals, 

 from which the fine calcigerous tubes radiate." 



Claypole in his above-mentioned work (1893.1, p. 608) after describing the structure 

 of the teeth of Dinichthys added that: "It is, in fact, a kind of transitional material between 

 typical bone and typical tooth." Moodie in an article on the nature of the primitive 

 Haversian system (1920) is of the same opinion, and thought that that the minute structure 



