66 TRANSACTIONS OF THE [NOV, 16, 



relations of this puzzling fossil and in throwing light upon a 

 primitive mode of spine formation within the phylum of fishes. 



In the first regard to return to the puzzling question * as to 

 what portion of a fish Edestus represented. 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 bod}- 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 in the first place such a 

 view would require E. lecontei to be the generalized rather than 

 the most specialized of the different types. And one w^ould 

 have to assume that this intermandibular, which in the onl^' 

 teleostome in which it occurs is in its basis a dermal structure, 

 has not merelj^ been paralleled by the far diflTerent jaw conditions 

 of an elasmobranch, but has been paralleled in a cartilaginous 

 tissue. Moreover, even granting the possibility of this, the com- 

 parison with an intermandibular element could not yet be made, 

 for in Edestus the shaft is segmented, and in Onychodus its basal 

 portion is unquestionably a single piece. 



If Edestus must now be excluded from the structures of the 

 mouth region, it would accordingly fall within the broad class of 

 spines. And this is the alternative which at the present day is 

 very generally accepted. But what manner of spine could it 

 have been ? On account of the segmented character of its shaft 

 Henry Woodward has compared it to the pectoral spines of the 

 Teleost Pelecopterus. Newberry on the other hand, reviewing 

 its characters in an extended memoir, decides with the clearest 

 and most convincing arguments that the spine was a median one, 

 that it was deeply implanted in the integument and that no fin 

 structures could have existed behind it ; it was therefore com- 

 parable to the spine series of the sting ra3^s, and its position ac- 

 cordingly^ might reasonably have been in the hinder trunk region. 

 But even this conclusion does not seem to the present writer 

 altogether a final one. For in Trycjon the spine series is ap- 

 parently compound, while in Edestus, the spine, although seg- 

 mented, is nevertheless a single one. There is, in other words, 

 no common spine shaft in the sting ray from which the denticles 

 take their origin. To make this comparison tenable it must, 

 therefore, be assumed that the individual element of the spine 

 series of Trygon represents a segment of Edestus. Of course 



*Cf. especially the views of Hitchcock, L. Agassiz, Henry Woodward and Newberry, 

 Newberry, op. cit. 



