BUNOTHERIA. 801 



MESODECTES Cope. 



(Expl. Survs. W. of lOOtli Mer., Lt. G. M. Wheeler); April, 1875. Syst. Catal. Vert. Eocene, New 

 Mexico, p. 30. laacis Cope, Paleontological Bulletin (August, 1873) No. 16, p. 3. Bulletin 

 U. S. Geol. Surv. Terrs., No. 1, 1874, p. 23. Ann. Kept. U. S. Geol. Surv., 1873 (1874), p. 470 

 ■ (nom. prieocc). 



This genus embraces at present but a single species, which is known 

 from numerous specimens discovered by the writer in the White River 

 Miocene formation of Colorado. From these it appears that Mesodectes is 

 closely allied to the Leptidis and Idops of Leidy, occupying a position 

 between these in the system. In Leptidis the last premolar is sectorial in 

 form, consisting of a single compressed longitudinal crest, without internal 

 tuberosity or cusp. In Idops the last premolar exhibits a structure similar 

 to that of the first ti-ue molar, viz, two exterior cusps and well-developed 

 third on tlieir inner side, thus giving a horizontal section of the tooth a sub- 

 triangular foi*m. In Mesodedes the last premolar possesses a single acute 

 cusp, as in Leptidis, with an internal cusp or heel homologous with that in 

 Idops. Such peculiarities are necessarily regarded as tangible definitions 

 of generic groups, and are such in this case, although they separate species 

 which have considerable resemblance in some other respects as far as known. 



The molars of the superior series have two exterior compressed conic 

 cusps and a stout subtriangular internal one. Behind the latter is a strong 

 cingulum, supporting a rudimental cusp behind and within the principal 

 one. Inferiorly there are three tubercular molars, of which the two anterior 

 are composed of two elevated cross-crests, which form partial V's, opening 

 to the inner side. The last premolar is larger than the true molars, and 

 supports three anterior conic tubercles, the inner and outer equal, and a 

 heel with a conic tubercle on the outer side. The number and character of 

 the teeth in front of this one are unknown. 



The posterior part of the cranium exhibits characters similar to those 



of Leptidis, but in the specimen the superior walls are wanting. The animal 



is not adult. The exoccipitals are distinct from the supraoccipital, displaying 



a wide, smooth sutural face. The mastoid is quite distinct, and is narrowed. 



Its superior portion enters into the posterior face of the skull, the union 



being formed by the posterior border of the squamosal. There are neither 



paroccipital nor mastoid processes, and the inferior extremity of the mas- 

 51 c 



