1881.] "*00 [Cope. 



Triisodon quivirensis, gen. et sp. nov. Char. gen. Derived from the 

 lower jaw. Probably only three premolars. True molars alike, consist- 

 ing of three anterior cusps and a heel. The cusps are relatively small and 

 the heel large. Of the former the internal is much smaller than the ex- 

 ternal, and the anterior is rudimental, being merely a projection of the 

 cingulum. The cutting edges of the large external 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 

 inner tubercle, so that the anterior part of the crown consists of a com- 

 pressed cutting cusp. The heel has two well-developed posterior cusps. 

 The third premolar has a similar principal trenchant cusp, but a smaller 

 heel. Canines large. 



This genus differs from HerpetotTierium and Ictops in the simplicity of its 

 fourth inferior premolar, and from Stypoloplius and Deltatherium in the rudi- 

 mental character of the accessory anterior cusps of the true molars, as well 

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

 lars, with the three similar true molars, separates it from Palceonyctis, and 

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

 from Amblyctonus and Periptychus. It is not possible to state whether 

 Triisodon must be placed in the Amblyctonidce or not, on account of the 

 absence of the superior molar teeth. 



This specimen of the type species of this genus is instructive as showing 

 the succession of premolar teeth. Both the third and fourth premolars 

 have temporary predecessors. The predecessor of the fourth premolar 

 differs 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 premolar is wanting, the roots only remaining in the jaw. 



The permanent third premolar was protruded before the permanent 

 fourth. Which temporary tooth of Triisodon is homologous with the 

 single one of the Mar&apialia pointed out by Professor Flower?* As the 

 additional permanent teeth of the placental Mammalia must have appeared 

 later in time than the one already found in the implacentals, they must be 

 those later protruded ; hence the fourth tooth in the jaw of Triisodon 

 must be regarded as homologous with the fourth premolar of a placental, 

 which is the last of that series to appear. If this be true, the tooth which 

 follows the shed tooth of the Marsupials is not the fourth premolar, as 

 supposed by Professor Flower, but the third premolar. This view is con- 

 firmed by the fact that the milk tooth displaced by the fourth tooth in 

 Triisodon resembles in all respects the true molars, just as the permanent 

 tooth occupying the same position does in Didelphys and some extinct 

 eocene genera. This goes to show that this tooth, permanent in marsu- 

 pials, is temporary in placentals, and that, in spite of its form in the for- 

 mer group, it is the fourth premolar, and not the first true molar, as sup- 

 posed by Professor Flower. Thus the posterior milk-molar of diphyodonts 

 is a permanent tooth in the Marsupialia. 



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



