118 Eelations of Edestus. 



somewhere on the median line, as a homologue of either the in- 

 termandibular arch of Onychodus, the dorsal spines of Chim- 

 aera and Hyhodus, or the caudal spine of Trygon. 



The suggestion of Miss Hitchcock, that Edestus is an inter- 

 mandibular bonv arch carr3'ing 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 

 Polypteriis, and has left abundant bones besides its interman- 

 dibular arch In Onychodus sigmoides of the Corniferous 

 limestone, and 0. Hophinsii, of the Chemung group, the 

 teeth are not anchylosed to the arch, are almost always found 

 detached, and the sides of the arch are compressed between the 

 extremities of the mandibles. In 0. Ortoni, of the Huron shale, 

 the teeth are implanted in the bony arch as a post is set in the 

 ground, and the arch is not distinctly impressed by the extrem- 

 ities of the mandibles. The type-specimen of 0. Ortoni is yet 

 unique, and we know nothing of the other parts of the fish 

 which bore it. It is of course not impossible that this singular 

 form of dentition might have been borrowed by some plagios- 

 tome which used it to accomplish a similar function ; but no 

 facts are yet known to warrant this supposition. 



Edestus Davisil is more like the intermandibular crest of 07iy- 

 cliodus than are the other species of the genus. It is much 

 more curved, and the arch of bone from which the denticles rise 

 is laterally compressed or longitudinally grooved. Taken by it- 

 self, it renders the suggestion of Miss Hitchcock quite plausible. 

 But it cannot be taken by itself, for wherever that species goes, 

 E. minor, E. Hemrichsii, and E. giganteus must follow, and 

 while we can imagine a fish ten feet long wath an arch of bone 

 like E, Davisii held between the extremities 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. Cer- 

 tainly such a monster would seem very much out of place in the 

 lagoons of the coal marshes. Again, E. Heinriclisii 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 addi- 

 tions at the acute end, wdiich must have been behind, and if it 

 was situated in the symphysis, the blunt rounded end would 



