MORRISON MAMMALS: PANTOTHHRIA 55 



but in 1887 he united these under the former name, recognizing the unity of the group 

 for the first and ahnost the only time. Osborn's first classification distributed the dryo- 

 lestids in four families, Peralcstidae, Kurtodontidae, Amblotheriidae, and Stylaco- 

 dontidae. placed in two different orders and, indeed, subclasses. Later in the same year 

 he suppressed the families Peralcstidae and Kurtodontidae and recognized the fact 

 that the dryolestids belonged in one order. In 1907 he finally united them in one 

 family" but his definition still perpetuated the deeply rooted but erroneous idea that 

 two quite different molar types are represented, one more normal, seen for example in 

 Amblotherium itself, and one styloid, essentially with but one cusp and one root. The 

 latest important classification, that of Gregory in 1922, unfortunately marked a step 

 backward, for he referred some dryolestids to the Amphitheriidae and for others resur- 

 rected the name Stylodontidae. The latter family is doubly invalid, for it was founded 

 in error, as Marsh quickly recognized, and was based on a preoccupied name. 



This long persistence of what may now easily be shown to be a fundamental error 

 is in no sense a reflection on the painstaking work on the group done by the authorities 

 mentioned. It is rather a tribute to the unusual difficulty of the whole problem, to the 

 absolute necessity of access to all the material and of removing concealing matrix when 

 necessary, and to the need for the best optical aids. Previous to the splendid coopera- 

 tion which now makes correction possible, no one worker had free disposal of the 

 necessary materials and equipment. Marsh worked under the most advantageous con- 

 ditions of the earlier students, and his final conception of the group was essentially 

 correct. 



The belief, then, has long been that certain genera 

 of upper Jurassic pantotheres, the so-called Stylodonti- 

 dae or Stylacodontidae, had much compressed, colum- 

 nar molars with connate roots. In fact, the number and 

 proportions of the cusps in these supposed genera are 

 the same as in the typical dryolestids, the trigonid is, if /^ftv ^ 



anything, longer, the talonid is present and similar in {^rm\ 



character, and the two roots are developed in exactly 

 the same way. Curiously enough, the t}^ical genera of 



the two supposedly distinct groups, Amblotherium and f ■^- ^4. Diagrams of dryolestid 

 ^ , , ^^ •' r, , e lower molar, essentially Amblolhe- 



btylodon, are synonymous. Both were perforce exam- „j,^ ^^ external view, partly 



ined with various cusps obscured by the matrix and buried in matrix, so-called Stylo- 



/l;w(5/o///^W«w was the internal, 5'/';'/(7a?o« the external, '^"^ o^ Stylacodon. B, internal 



aspect of the same genus. "^y- ^' '7"^;;^^^^ f ^,^"°"' f ""^"^ 



» o protoconid. D, relation of roots 



The discovery of this and of other similar errors (solid black) to crown (outline). 



'* Ostwm, 1907, p. 23. He called the family "Amblotheriidae Osborn or Stylacodontidae Marsh." 

 Marsh did not propose a family Stylacodontidae, however, and Amblotheriidae Osborn is identical with 

 the earlier Dryolestidae Marsh. Osborn apparently abandoned the latter name because of his belief in 

 the probable synonymy of Dryolesles and Phascolestes. Direct comparison shows these genera to be dis- 

 tinct, and there is no reason why the first name applied to the family, Dryolestidae, should not be 

 retained. 



