ON THE NATURE OF EDESTUS AND RELATED FORMS. 283 



covery of Campodus by Prof. E. H Barbour had thrown new light on the whole 

 matter, they were positively determined as arched or coiled series of symphysial teeth 

 belonging to cestraciont sharks. The evidence for this was presented by the writer 

 before the Denver meeting of the American Association for the Advancement of 

 Science, and in the published abstracts of that paper (:01, p. 795) the arrangement 

 of the symphysial series in Campodus variabilis (N. and W.) was briefly described. 

 The most recent contribution to the literature of this subject is contained in the cur- 

 rent volume of the Bulletin of the Museum of Comparative Zoology (Eastman, :02), 

 where the same interpretation is still further elaborated. 



It will be seen from the foregoing review that authors have been about equally 

 divided between the two leading theories of Edestus, one of which ascribes them to 

 the jaws, the other to the external dermal armor of a shark or skate, and that until 

 recently, confirmation of either by direct evidence has been wanting. Attention 

 is now invited to lately acquired evidence of this nature, which is alone competent 

 to decide the issue. This evidence is furnished by three remarkable specimens of 

 Campodus variabilis (N. and W.) from the Coal Measures of Kansas and Nebraska, 

 which together afford a complete understanding of the dentition in the group of ces- 

 traciont sharks to which this species belongs. The thesis is maintained that the 

 fused segments of Edestus, Campyloprion and Helicoprion are veritable teeth cor- 

 responding to the symphysial series of Campodus, which are enormously enlarged 

 as compared with those of Cestracion and other recent sharks; and also that these 

 four Carboniferous and Permian genera together constitute a remarkable series, in 

 which the progress of evolution is readily traceable. Beginning with Campodus, 

 we are enabled to note in the species of Edestus and Campyloprion the progressive 

 stages by which the typical orodont dentition of the Lower Carboniferous passed 

 into the excessively modified spirals of Helicoprion before the close of the Palaeozoic. 



Before proceeding to develop our present thesis, however, we may briefly con- 

 sider the arguments which have been urged for excluding Edestus and related forms 

 from the mouth region. The question as to what portion of a fish is represented by 

 Edestus has been ably discussed by three recent writers, Newberry, Dean, and Kar- 

 pinsky. From Dean's concise statements ('98, p. 66) we quote as follows: 



"Accepting as a logical necessity that the fossil is elasmobranchian, the choice 

 of its location cannot be a wide one; it must have belonged either in the mouth region 

 or on the body surface, in the latter case evidently as a spine. As to its belonging 

 in the mouth region the segmented nature of its base has ever precluded the view 

 that it was in any way connected with Meckelian or palato-quadrate structures. And 

 as an intermandibular element its position is even less plausible. For . . . one 



