272 THE rUERCO FAUNA. 



series of modifications of the dental characters, proceeding from the simple 

 to the more complex, may be constructed as follows: 1. Mesonyx; 2. Dissa- 

 cus; 3. Sarcothraustes ; 4. Triisodon; 5. Amhlydonus ; 6. Deltatherium. The 

 first three belong to the Mesoni/chidce, as distinguished by the form of the 

 tarsal articulations. Whether Triisodon must be arranged with Amhlydonus 

 or not, cannot be ascertained until the foot structure is known. 



As the number of the inferior premolars in three of the species now 

 referred to this genus is unknown, it is possible that some of them ma}' be 

 hereafter referred to Ictops. 



Triisodon quivirensis Cope. 



Loci Bupra citati. 



Plate XXV. c ; fig. 2. 



Represented by both rami of the mandible, which exhibit alveoli or 

 crowns of all the teeth excepting the incisors. Size about that of the wolf. 

 Inferior canine directed upwards, its section nearly elliptic; a faint posterior, 

 no anterior cutting edge. Fourth premolar rather large, with an anterior 

 basal cingulum whicli is angulate upwards, and is not continued on the 

 inner side of the crown. Cusps of the heel each sending a ridge forwards, 

 the internal lower, obtuse, and descending to base of inner side of large 

 cusp; the external larger, with an acute anterior cutting edge continuous 

 with the cutting edge of the large cusp. True molars with an external, but 

 no internal basal cingulum. Border of heel with one large and three smaller 

 tubei'cles, the former with, the latter without, anterior cutting edge. En- 

 amel of all the teeth nearly smooth. All the cusps are rather obtuse. 



Measurcmenf.1. 



L<-n<;tIi of inferior molar series 080 



Long diameter of base of canine 013 



Length of true moiar series 044 



Lentil of ba-se of Pm. iv 016 



Elevation of crown of do 014 



Length of bnso of M. ii 010 



Width of do. in front OH 



Elevation of do 014 



The measurements of the jaw are not given, as the animal is not adult, 

 the last molar not being yet fully protruded. 



