MORRISON MAMMALS: TRICONODONTA 33 



TvPE. — TriconodoH Owen. 



Distribution. — Purbeck beds of England and Morrison formation of Wyoming. 



Trioracodon'^\xxv^%o\\ 1928 

 1928. Trioracodon, Simpson, Cat. Ales. Mam. Brit. Mus., 88. 



Definition. — Dental formula \\ C\ V\ M'g. Anterior and posterior cusps of 

 lower molars a little lower relative to the midcusp than in Triconodou. Cheek tooth 

 series closed, molars relatively large. M* reduced and bicuspid. 



Type. — Trioracodon ferox (Owen). 



Distribution. — Purbeck beds of England and Morrison formation of Wyoming. 



Marsh supposed (1887) that there were three upper Jurassic genera of tricono- 

 donts, distributed as follows: 



Purbeck Morrison 



Triacanthodon 



Triconodou Triconodon 



Priacodon 



The dental formulae were supposed to be : 



Triconodon: P4 M3 

 Triacanthodon: P4 M4 

 Priacodon: P3 M4 



The first two formulae were assigned by Owen, the last by Marsh. Osborn ( i888a) 

 questioned this arrangement and gave reasons for believing that Triacanthodon is a 

 synonym of Triconodon and that this genus had the formula P4 M4. The writer 

 (19253) showed that in the American species referred by Marsh to Triconodon there 

 were never more than three molars, and largely on this basis he continued to maintain 

 the separate status of Triacanthodon. This view is now acknowledged to have been 

 erroneous. Study of the original English material has shown beyond question that 

 Osborn was right in considering Triconodon and Triacanthodon as synonymous and 

 in assigning to Triconodon the formula P4 M4. It has further been found, however (see 

 Simpson 1928B, p. 88 seq., for details), that there does occur in the Purbeckian an 

 allied genus with a formula of P4 M3 and with various other differences from Tricono- 

 don, to which the name Trioracodon has been applied. The American species "Tri- 

 conodon" bisulcus belongs to this genus, being especially close to the Purbeckian Trio- 

 racodon oweni Simpson. The result of this change is that the application of the various 

 names is modified but that the essential facts remain the same as suppsed by Marsh, 

 as may be seen in the following tabular presentation : 



Purbeck Morrison Cheek Teeth 



Triconodon P4 ^\ 



Trioracodott Trioracodon P4 M3 



Priacodon P3 M4 



