NO. 1884. A NEW SPECIES, EDESTUS MIRU8—HAY. 35 



teeth were pushed forward and out of the mouth, but instead of 

 remaining free from the adjacent teeth and falUng away, their bases 

 cohered to form a shaft. In the species before us the outer end of the 

 lower shaft was directed forward and downward, while the upper 

 shaft was directed forward and upward. It is entirely improbable 

 that the tooth found at the outer end of each of these shafts was the 

 first tooth the animal possessed. One must therefore believe that, 

 although the outer segments of the shaft appear to be very solidly 

 united, those of the older teeth ditl, in succession, lose their hold on 

 the younger ones and become detached. 



This Iowa specimen enables us to determine which end of the shaft 

 is the anterior and in what order the new segments were added, and 

 here the opmion hekl by most -sviiters is reversed. That end wliich 

 in a former paper the writer regarded as the front end is in 

 reality the hinder end. The bases of the crowns of the teeth are 

 drawn out backward, not forward. The tooth which is seen at the 

 left end of the figure of Edestus crenulatus ' is not the last tooth that 

 was formed, but the first, at least the first of those present. In his 

 description of the type species of the genus, E. vorax, Doctor Leidy ^ 

 correctly judged which was the anterior end of the fragment that he 

 had, but he supposed that it was a part of the maxilla of some fishhke 

 animal. Dr. J. S. Newbeny,^ in his description of E. giganteus, 

 stated that the teeth, or denticles, were prolonged backward and 

 downward into a simple point. In this opinion, as shown by the 

 specimen at hand, he was correct. However, on the preceding page 

 Newberiy ^^Tites: ''Agam, E. Jieinrichsi 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 to the acute end, which must 

 have been behind," etc., a statement that contradicts the one just 

 referred to regarding the direction in wliich the enamel is prolonged. 

 In describing the manner of growth of the mass,'' he said: "The 

 numerous disconnected segments of Edestus heinricJisi, furnished me 

 by Mr. Butts, seem to prove conclusively that the spine was elongated 

 by the addition of a sheath, carrying a denticle, to the extremity and 

 underside of the })reexisting series." It is to be recollectetl that 

 Doctor Newberry believed that the mass was a dorsal defensive 

 spine. 



Like Newberry, the present writer held that the last -formed chan- 

 neled tooth base was apphed to the border of the shaft opposite (he 

 one bearing the teeth; but now it is necessary to believe (hat the 

 newer tooth base was laid down in the trough of the one iuimodiately 



> Proc. U. S. Nat. Mus., vol. 37, 1909, pi. 12, fig. 1. 

 ' Journ. Acad. Nat. Sci. Phila., scr. 2, vol. 3, p. ItiO. 

 « Pal. Fishes, N. A., p. 225. 

 * Idem, p. 223. 



