IIG delations of Edestus.- 



tus Davisii. It is the impression of a bony arch about four in- 

 dies long, on the convex border of which are set fourteen acute,, 

 compressed, lancet-shaped, crenulated denticles. It is more 

 curved than the other described species of Edestus, but is so like 

 them that Dr. Woodward seems to have been fully justified in 

 placing it in that genus. In his discussion of the structure and 

 relations of this fossil, Dr. Woodward compares it with the seg- 

 mented s])ines <>f PeJecopferits, Cope, from the Cretaceous rocks 

 of Kansas, and is thereby led to consider it a ]iectoral defensive 

 si)ine. There are, however, some points in the structure of this 

 and other species of Edestns, which will be alluded to further 

 on, and which make it difficult for us to accept this conclusion. 



In August, 1887, Miss Fanny K. M. Hitchcock, an earnest 

 and accomplished student of comparative anatomy, read a paper 

 before the Biological Section of the American Association '• On 

 the homologies of the so-called spines of Edestus,^'* in w^hich she 

 suggests''that Edestus was an intermandibular arch of bone car- 

 rying teeth, and most like the dentigerous arch which was held 

 between the extremities of the mandibles in the great Crossop- 

 terygian Ganoid, Onycliodus sigmoides, found in the Cornife- 

 rous limestone of Ohio, and desci'ibed by the writer in the Palae- 

 ontology of Ohio, Vol. J, p. 299, PL XXVI, figs. 1-5, PI. 

 XXVII, figs. 1, 2. There are pei'haps no facts which disprove 

 this hypothesis, and it is worthy of resi)ectful consideration, but 

 I would suggest that Onycliodus was a highly organized Ganoid 

 and very widely separated zoologically from Edestus, which 

 must have been a Plagiostome. At least, unless the skeletons of 

 huge fishes like Edestus giganteits were cartilaginous, we should 

 find their bones in the rocks where their spines are so numer- 

 ous. 



The structure and probable functions of Edestus have been 

 discussed by the writer at some length in the notes on E. Hein- 

 richsii, in the Geology of Illinois, Vol. IV, p. 350 ; and the 

 conclusion is there reached that it was not a jaw, but the defensive 

 dorsal spine of a plagiostomous fish. The considerations which 

 lead me to this conclusion are briefly as follows : 



1. Although the denticles which crown the convex border of 

 Edestus have the general form and crenulation of the teeth 

 of Carcharodon and Remipristis, their structure is in many re- 

 spects quite different, viz., the teeth of none of the sharks are 



