1893.] NATURAL SCIENCES OF PHILADELPHIA. 203 



tion anterior to the second true molar to represent the long hori- 

 zontal production in Dibelodon tropicus, Mastodon americanus, etc. 

 The symphysis is also much compressed above, so that the spout is 

 narrow. The extremity of the spout is produced and contracted, 

 and slightly recurved at the extremity, and there issues from the 

 right side a well developed, vertically compressed mandibular tusk. 

 On the left side is the empty alveolus of its counterpart, which was 

 of much larger diameters than that of the right side. The portions 

 of superior tusks found in California are stated by Leidy to possess 

 an enamel band. As there are mandibular tusks, I refer the species 

 to Tetrabelodon rather than to Dibelodon as heretofore. 



The contracted symphysis steeply descending forward and 

 expanding downward at the base from a posterior elevation, dis- 

 tinguishes this species from any of those of the genera Tetrabelodon, 

 Dibelodon or Mastodon known to me. Hence there is no question 

 of its difference from T.productus Cope, with which it was identified 

 by Leidy, 1 where the symphysis is flat and much longer. In 

 Dibelodon cordillerantm, according to D'Orbigny and Burmeister, 

 the symphysis is not elevated behind, is produced and decurved 

 at the extremity, and has a wide spout — all characters quite differ- 

 ent from what is seen in T. shepardii. The symphysis is totally 

 distinct in D. tropicus, where it is toothless and not decurved, 

 and the last true molar is much more complex in the latter. 

 I formerly identified the species so abundant in the Equus Beds 

 of the valley of Mexico with this species. This determination 

 must now be reconsidered, since the form of the mandibular sym- 

 physis is entirely different ; there is no mandibular tusk, and the 

 last lower molar is not identical in form, 2 though I formerly thought 

 it not so different as to preclude the possibility of the species being 

 identical did no other differences exist. The differences observed in 

 this tooth may, however, be of specific value. The relative width 

 of the crown is greater, especially anteriorly, and the lateral borders 

 of the latter are not parallel, as in T. shepardii, but converge pos- 

 teriorly, as in M. precursor. The lobes of the cross-crests are 

 opposite, as in the latter species. From M. praecnrsor this tooth 

 differs in the more tubercular and less crest-like character of the 

 cross-crests, and in their more profound division on the median line, 

 especially in the third and fourth crests. M. praecursor is zygolo- 



1 By reference to it of specimens of T. productus from Santa Fe, New Mexico, 

 in Report U. S. Geol. Surv. Terrs. I, 1873, p. 5. 

 ' Proceeds. Am. Philos. Soc, 1884, p. 5. 



