270 THE PUEECO FAUNA. 



TRIISODON Cope. 



American Naturalist, 1881, p. 667, August (July 27). Proceed. Amer. Philos. Soc, 1881, p. 485. 



The type of this genus is only known from portions of lower jaws; 

 some of these include the entire dentition, with unimportant omissions. 



True molars alike, consisting of three anterior cusps and a heel. The 

 cusps are relatively small and the heel large. Of the former the internal 

 is mucli smaller than the external, and the anterior is rudimental, being 

 merely a projection of the cingulum. The cutting edges of the large ex- 

 ternal cusp are obtuse. The heel is basin-shaped, and its posterior border 

 is divided into tubercles, of which the external is a large cusp. The fourth 

 premolar has no anterior or posterior inner tubercles, so that the anterior 

 part of the crown consists of a compressed cutting cusp. The heel has 

 two well-developed posterior cusps. The third premolar has a similar prin- 

 cipal trenchant cusp, but a smaller heel. Canines large. 



This genus differs from Herpetotherhim and Ictops in the simplicity of 

 its fourth inferior premolar, and from Stypolophus and Deltatherium in the 

 rudimental character of the accessory anterior cusps of the true molars, 

 as well as in the three premolars. The rudimental anterior cusp of the true 

 molars, with the three similar true molars, separates it from Palceoni/ctis, and 

 the presence of a conic inner cusp of the same indicates it as different from 

 Amhlyctonus and Periptychus. It is not possible to state whether Triisodon 

 must be placed in the AmUyctonidce or not, on account of the absence of the 

 superior molar teeth of the typical species T. quivirensis. 



This specimen of the type species of this genus is instructive as show- 

 ing the succession of premolar teeth. Both the thii-d and fourth premolars 

 have temporary predecessors. The predecessor of the fourth premolar dif- 

 fers much from it in form, and is essentially identical in all respects with 

 the true permanent molars. The crown of the predecessor of the third i)re- 

 molar is wanting, the roots only remaining in the jaw. 



The i)ermanent third premolar was protruded before the permanent 

 fourth. Which temporary tooth of Triisodon is homologous with the single 

 one of the Marsupialia pointed out by Professor Flower!' As the addi- 



■ Transactions of the Royal Society, 1867, p. 631. 



