0)1 the Slrnvtvrc- and 117 



symmetrical, one fMce is flattened and the other is more or less 

 arched ; wiiile the denticles of Bde,stu,s are equally arched on 

 both sides. 



'2. The teeth of sharks, while having- an enameled crown, have 

 a tnmid, bony base attached by ligament to the cartilaginous 

 jaws, and separating readily from them ; hence they are rarely 

 found in place in the fossil state. The denticles of Edestus, on 

 the conti'ary, are firmly attached to the bony arch from which 

 they rise. 



o. The form of these fossils is quite unlike that of any jaw of 

 fish, reptile, or mummal known ; being roughly rounded at the 

 base, the opposite extremity flattened and bordered on one side 

 by a sharp edge, on the other by crenulated denticles, one of 

 which is terminal. 



4. The rounded roughened base proves that this organ could 

 not have been articulated with any bones and scarcely \'/ith car- 

 tilages ; else we should have some evidences of coadaptation. In 

 this respect it resembles most the dorsal spines of sharks and 

 skates, which are implanted in the integuments of the back and 

 have a roughened, base and bony structure, with various forms 

 of enameled denticles on one margin. 



5. If the spines of Edestus were attached to the head, as mod- 

 ified jaws and the homologues of the rostrum of Pristis. the base 

 would present some evidence of anchylosis with the bones or car- 

 tilages of the head ; whereas it is rounded as though it had been 

 buried in soft tissue. Again, the rostrum of Pristis is only par- 

 tially ossified, while the spines of Edestus are composed of dense 

 bone ; and, further, the denticles of the rostrum in Pristis are 

 set in alveolar cavities, from which they escape and are scattered 

 about in the decay of the animal. We often find these denticles 

 in the Cretaceous marls, but almost always isolated, like the 

 sharks' teeth which occur with them. On the contrary, the 

 denticles of Edestus are inseparably united with their bony 

 bases, and they are perfectly preserved together. P'inally, if 

 each spine of Edestus was one of a pair attached to the snout, 

 like the rostrum of Pristis, Xiphias, or Ccelorhynchus, they 

 must have been entirely separated, for they bear no marks of 

 contact, and they would certainly have been unsymmetrical. We 

 are therefore, driven by the bilateral symmetry of Edestus to 

 conclude that it was not one of a pair, but that it stood alone 



