VERTEBRATES. 351 



conclusion, and indicates that it was rather a defensive spine. Although the 

 denticles which crown its convex margin have the general form and crenulation 

 of the teeth of Oarcharodon or Hemipristis, their structure is, in many respects, 

 quite different. 



1st. The teeth of none of the sharks are symmetrical, but the anterior face 

 is flattened, and the posterior is more or less arched, while on the denticles of 

 the specimen before us the two sides are equal. 



2d. The jaws of sharks are cartilagenous, holding the bony and enameled 

 teeth only by ligamentous attachments, so that in the fossil state the jaws have 

 usually quite disappeared, the teeth being scattered about in all directions, 

 whereas in Edestus we have a mass of dense bone to which tooth-like denticles 

 are united by a firm, bony anchylosis. 



3d. The form of this fossil, as shown by the nearly complete specimen before 

 us, is wholly unlike that of any jaw of fish, reptile or mammal known, being 

 roughly rounded below, above terminating in an acute point, its upper portion 

 flattened, smooth, even-polished, evidently never having been covered by the 

 integuments, and is bordered on one side by a sharp cutting edge, on the other 

 by crenated denticles. 



. 4th. The rounded, roughened base proves that it could not have been articu- 

 lated with any bones, and scarcely with cartilages, else we should have some 

 evidences of co-adaptation. In this respect, it resembles most the dorsal spines 

 of sharks and skates, which are implanted in the integument of the back, have 

 a roughened base and a bony structure, with various forms of enameled denti- 

 cles on one margin. 



It is apparent that this fossil is genetically identical with that exhibited at 

 the ninth meeting of the Am. Ass. by Prof. Hitchcock, and of which a better 

 figure is now given than any heretofore published. That specimen was found 

 in the coal of Parke Co., Ind., and when exhibited to the Am. Ass. was con- 

 sidered by Prof. Agassiz as a jaw ; one of a pair placed on the sides of the 

 head, and compared to the embryonic condition of the saw of Pristis. There 

 are, however, some features in the specimen before us, which seem to militate 

 against that conclusion : 



1st. The tissue of the rostrum of Pristis is only partially ossified, while this 

 is all dense bone. 



2d. The denticles of the rostrum in Pristis are set in alveolar cavities, while 

 in this fossil they are inseparably united with the bony mass without sockets. 



3d. If the fossil was the homologue of the rostrum of the saw -fish, the base 

 would have presented some evidence of articulation with the bones or cartilages 

 of the head, whereas it is rounded as though it had been completely buried in 

 soft tissue. 



