Marsh Collection^ Peabody Museum. 355 



has a Y-shaped pattern. One arm of the Y extends forward 

 and inward to join the base of the trigon and the other inward 

 and backward to the posterior cusp. The internal cusp is 

 relatively small and conical, and situated directly opposite the 

 large external one. In front of this, between it and the 

 internal cusp of the trigon, is a deep notch through which the 

 valley opens internally. The posterior cusp of the heel is 

 small and indistinct ; it is situated upon the posterior rim of 

 the central valley, more to the inner than to the outer side ; it is 

 connected with the outer Y-shaped cusp by a low^ ridge, and is 

 separated from the inner cusp by a notch. The crown of the 

 fourth premolar is nearly like that of the true molars, the 

 only noticeable diiference in its structure being the absence of 

 the anterior cusp of the trigon, together with the smaller size 

 and more posterior position of the interior trigonal cusp. The 

 chief characteristics of these teeth are seen in the broad heel 

 as compared with the trigon, as well as the slight elevation 

 and distinctness of the cusps of the latter. 



Description of the Type of Mescicoclon speciosus. — The 

 specimen upon which this genus and species were founded 



110 



Figure 110. — Lower jaw of Microsyops elegans Marsh (type of Mesacodon 

 speciosus Marsh) ; yiewed from above ; two and one-half times natural size. 



consists of a well-preserved lower jaw, figure 110, of the right 

 side, lacking the condylar, coronoid, and angular portions. 

 The last molar is missing, as well as the canine or second pre- 

 molar and the crown of the large incisor. After careful com- 

 parison with Microsyops elegans, I can not discover any differ- 

 ence between the two. The teeth are very nearly of the same 

 size and, as far as ascertainable, the crowns of the molars and 

 premolars are constituted in exactly the same way. I do not 

 hesitate, therefore, to refer them to the same genus and species. 

 The additional information furnished by this specimen per- 

 mits an accurate determination of the entire dentition of the 

 lower jaw. The enlarged incisor is implanted by a distinct 

 root and was not, therefore, of persistent growth ; its position 

 is procumbent, being directed much forward and a little up- 

 ward. Most of the crown is broken away, but enough remains 

 to show that the enamel w^as not limited to the anterior face 

 of the tooth, as in CJieironiys and the Rodentia, but invested 



