ON THE NATURE OF EDESTUS AND RELATED FORMS. 285 



auch der Vermuthung, die Spirale hatte, sich in der Mundhohle befunden, den Boden, 

 sowohl in dem Sinne, wie er anf anglich von Leidy und Hitchcock angenommen wurde, 

 sondern auch im Sinne eines ' intermandibular arch/ wie Miss Hitchcock vermuthete, 

 was am besten auf Edestus davisii, d.h. auf die Gattung Hehcoprion anwendbar 

 schien. . . . Ferner kann man noch hinzufugen, dass auch der Bau der Basis der 

 Spiralwindungen der Hypothese von Miss Hitchcock widerspricht. 



" Nur die Schlusse : (1) dass die Edestiden in der That zu den Elasmobranchiern 

 gehoren, (2) dass die Spirale von Helicoprion und das entsprechende Organ von 

 Edestus nicht frei gestanden hat, d.h. dass ihre Segmentbasen in die Weichtheile 

 des Thieres eingebettet gewesen sind, (3) dass die besprochenen Organe in die Medi- 

 anebene des Thieres zu verlegen sind, und (4) dass der grosste Theil der Helicoprion- 

 spirale, sowie das entsprechende Organ von Edestus ausserlich angebracht gewesen 

 ist, konnen als ausgemacht gelten. Alle iibrigen Schlussfolgerungen in Betreff der 

 Morphologie der Edestiden dagegen werden vermuthlich noch lange in den Augen 

 der Forscher als unbewiesene Hypothesen dastehen. 



"Und es liegt keineswegs ausserhalb des Bereichs der Wahrscheinlichkeit, dass 

 in der Folge solche den Edestidenresten ahnlichen Organe entdeckt werden, von 

 denen wir im Augenblick noch keine Ahnung haben." 



It is thus clear that the objections to referring Edestus-like structures to the 

 mouth region are altogether of a theoretical nature, due partly to a priori conceptions, 

 and in still larger measure to ignorance of the extent to which certain peculiarities 

 in the growth of teeth prevailed amongst Palaeozoic sharks. For instance, we have 

 only recently gained the information, through a studj^ of such forms as Protodus, 

 Periplectrodus, Campodus, and certain eochliodonts, that there were not only Palaeo- 

 zoic sharks with crushing teeth, but also others with sharp, piercing teeth, which 

 were never shed, but became fused into whorls as the animal grew. And without 

 a knowledge of the symphysial dentition in Campodus, certainly no one could have 

 suspected such enormous disparity in size to exist between this and the lateral series. 

 According to our thesis, we must look to this latter circumstance for a key to the 

 solution of the whole problem of the "Edestidae." 



With the enlightenment furnished by Campodus in this respect, we are enabled 

 to appreciate the force of the following considerations, namely, the histological struc- 

 ture of Edestus and related genera is entirely in accord with their interpretation as 

 teeth; the segments in all these forms exhibit the same general pattern, are similarly 

 arranged, similarly fused, and similarly nourished by a continuous nutritive canal. 

 And as a logical necessity from their being homologous structures in every respect, 

 it follows that they were all located like Campodus in the mouth cavity. For the 



