Apbil 30, 1915} 



SCIENCE 



659 



Trachodon can be placed on a soTind footing, 

 there is now reason for believing this genus 

 is not present in the Lance formation, as indi- 

 cated by the fewer number of teeth in all of 

 the known specimens from the Judith River, 

 Belly River and Two Medicine formations. 



In the U. S. National Museum collections 

 there are several complete dentaries of the 

 Lance trachodonts in which the vertical rows 

 of teeth vary from 52 to 57 in number. All 

 of those in the collection from the other for- 

 mations mentioned above have fewer rows, 

 varying from 39 to 47. When it is known 

 that each vertical row has from six to ten 

 teeth, the difference in the total number is 

 considerable. 



The same condition prevails in the few 

 maxillffi available. It thus seems that the earli- 

 est known trachodonts as in the more primi- 

 tive Ceratopsians, have a smaller number of 

 teeth, so that now it may be safely asserted 

 that one of the marked phases in the speciali- 

 zation of the members of this group in suc- 

 cessive geological periods is a progressive in- 

 crease in the number of teeth in the dental 

 magazines. 



Since it is now known that the genua 

 Trachodon is based upon specimens from the 

 Judith River formation, and that all available 

 trachodont material from that and equivalent 

 formations shows a reduced dentition, this 

 smaller number of teeth would in itself con- 

 stitute a difference sufficient to restrict the 

 genus Trachodon to species from the Judith 

 River formation. 



This difference I have no doubt will be aug- 

 mented by other characters when sufficient 

 material is obtained for comparison. I would 

 therefore restrict the genus Trachodon to 

 Judith River species. 



This leaves the Lance trachodonts without 

 generic designation, and it wiU on that ac- 

 count be necessary to revive one of the older 

 generic terms, but I find that had been done 

 before the appearance of Hatcher's article. In 

 1901* Lucas called attention to the identity 

 of the type of Thespesius occidenialis Leidy 

 with the homologous parts of Claosaurus 



*F. A. LueaSj " Paleontological Notes," Sci- 

 ence (2), Vol. 12, 1900, p. 809. 



anneciens Marsh, and " that consequently this 

 Dinosaur must be known by Leidy's name." 



In 1902^ Hay, upon the authority of Lucas, 

 made Claosaurus anneciens a synonym of 

 Thespesius occidenialis, including under the 

 same genus the Niobrara species 0. agilis, but 

 this proposed change in nomenclature has been 

 entirely ignored by paleontologists in subse- 

 quent work. 



I have recently compared the types of 

 Thespesius occidenialis Leidy and Claosaurus 

 anneciens Marsh and can testify to the close 

 similarity of the homologous bones. The in- 

 adequacy of the type material upon which 

 Thespesius is based (two caudal centra and a 

 proximal phalanx) is fully recognized, but that 

 these pertain to a trachodont dinosaur there 

 can be no doubt. It is now positively known 

 from the geological mapping done in recent 

 years, in the locality where this material was 

 obtained, that the specimens came from the 

 Lance formation on the Grand River in what 

 is now the state of South Dakota. Despite 

 the meagerness of the material upon which it 

 is founded, it seems to me that Thespesius, 

 being the older term, is the logical choice of 

 names for the designation of the trachodont 

 dinosaurs from the Lance formation. 



While it can not be positively demonstrated 

 that occidenialis and anneciens are identical, 

 it is equally true that they can not be shown 

 to represent distinct species. Since the local- 

 ities from which the type specimens came are 

 not far apart geographically, it appears most 

 probable, however, that they do represent one 

 and the same species. I would therefore en- 

 dorse the use of Thespesius occidenialis as first 

 proposed by Lucas. 



The chief points that I have attempted to 

 bring out in the preceding lines may be 

 summed up as follows: 



1. That the trachodont dinosaurs of the 

 Judith River and equivalent formations have 

 fewer vertical rows of teeth in the jaws than 

 those from the Lance. 



2. That this feature constitutes a sufficient 



5 O. P. Hay, ' ' Bibliography and Catalogue of 

 the Fossil Vertebratea of North. America," Bull. 

 No. 179, U. S. Geol. Surv., 1901, pp. 502-503. 



