110 THE NEOG^IC REALM. [CHAP. 



evidence of a single specimen from Ecuador under the name of 

 Hyracodon fuliginosus, but whose affinities were not determined 

 till 1895, when an example of a second and larger species was 

 procured from Bogota, belongs to the sole surviving genus of a 

 group of small marsupials which occur abundantly in the Santa 

 Cruz beds, and were till quite recently regarded as extinct. Un- 

 fortunately the name Hyracodon has been previously employed 

 to designate an extinct ungulate, and it has accordingly been 

 replaced by Ccenolestes. The essential characteristic of this group 

 of marsupials, is that while their upper dentition is of a poly- 

 protodont type, that of the lower jaw is very similar to the 

 diprotodont modification. In the living species, for instance, 

 there are four pairs of small upper incisors of a normal type, 



FlG. 26. FORE PART OF THE RIGHT HALF OF THE LOWER JAW OF 



Acdestis oweni. (Much enlarged.) 



The first tooth on the right is the first incisor, and the one on the extreme 

 left the first molar. 



followed by a large canine, while in the lower jaw, as shown in 

 the accompanying figure of that of one of the extinct forms, there 

 is a large pair of horizontally projecting incisors, succeeded by 

 several minute functionless teeth, of which the first three represent 

 the second and third incisors and the canine. In all the forms 

 the molar teeth have quadrangular crowns, surmounted by four 

 blunt tubercles, somewhat resembling in structure those of certain 

 ungulates, and thus totally different from the triangular and 

 sharply-cusped molars of the opossums and other polyprotodonts. 

 In the living forms, as well as in certain fossil kinds (Epanorthus, 

 Decastis and Acdestis] from the Santa Cruz beds, the last premolar 

 tooth, as shown in the figure of the jaw of Acdestis, is of normal 

 dimensions : and these forms may consequently be grouped in a 

 single family, under the name of Epanorthidce. In another group, 



