284 ON THE NATURE OF EDESTUS AND RELATED FORMS. 



would have to assume that this intermandibular, which in the only teleostome in which 

 it occurs is in its basis a dermal structure, has not merely been paralleled by the far 

 different jaw conditions of an elasmobranch, but has been paralleled in a cartilaginous 

 tissue. Moreover, even granting the possibility of this, the comparison with an inter- 

 mandibular element could not yet be made, for in Edestus the shaft is segmented, 

 and in Onychodus its basal portion is unquestionably a single piece." 



Newberry's line of argument ('88, p. 118), in his discussion of the same ques- 

 tion, is as follows: 



"The suggestion of Miss Hitchcock, that Edestus is an intermandibular bony 

 arch carrying teeth, is not incompatible with its bilateral symmetry; but we here 

 meet the difficulty already suggested, that Onychodus, the only fish known which 

 had such an intermandibular arch of bone, was a scaled ganoid allied to Polypterus, 

 and has left abundant bones besides its intermandibular arch. ... It is of course 

 not impossible that this singular form of dentition might have been borrowed by some 

 plagiostome which used it to accomplish a similar function; but no facts are yet known 

 to warrant this supposition. 



"Edestus davisii is more like the intermandibular crest of Onychodus than 

 are the other species of the genus. . . . Taken by itself, it renders the suggestion of 

 Miss Hitchcock quite plausible. But it cannot be taken by itself, for wherever that 

 species goes, E. minor, E. heinrichi, and E. giganteus must follow; and while we can 

 imagine a fish ten feet long with an arch of bone like E. davisii held between the ex- 

 tremities of the mandibles, it requires a much greater stretch of the imagination to 

 conceive of a shark of such size that this relatively insignificant organ was twenty 

 inches long and seven or eight inches wide. Certainly such a monster would seem 

 very much out of place in the lagoons of the coal marshes. Again, E. heinrichi is 

 nearly straight, a foot long, rounded and massive at one end, thin and acute at the 

 other; but the succession of denticles was by additions at the acute end, which must 

 have been behind, and if it was situated in the symphysis, the blunt rounded end 

 would have formed the apex of the arch of the lower jaw — a condition of things 

 scarcely comprehensible. 



"If now we transfer this spine to the position of the post-dorsal fin, and bury 

 it in the soft parts all except the denticles, the elongation backward by the successive 

 addition of sheaths and denticles becomes intelligible and natural." 



Karpinsky's reasons ('99, pp. 446 and 469) for excluding the fused segments 

 of Edestus and Helicoprion from the mouth cavity are very positively stated, as will 

 be seen from the following extract: 



"Die ausseren Merkmale der von uns besprochenen Helicoprion-Reste entziehen 



