MORRISON MAMMALS: PANTOTHERIA 



91 



Y PM- n?^? 



Fig. 39. Docodon. Crown 

 view of typical right lower 

 molar. Fifteen times natu- 

 ral size. 



anterior slope it bears a stout but much smaller cusp which, by comparison with the 

 more primitive Pcra'tocyuodou, appears to be the homologue of the paraconid, more 

 external here than in other pantotheres. Also along the outer border, but lower and 

 more separate from the pr<' is the external talonid cusp. 1 1 has the position of the hypo- 

 conid of later mammals. The internal face of the tooth is also convex and bears a 

 similar row of marginal cusps. The highest of these is a little 

 posterior to the midline and only about half as high as the pr"*. 

 It appears to be the me*". On its posterior slope, only a little 

 lower than it and hardly smaller, is the internal talonid cusp. 

 This has about the position of the talonid cusp of the dryoles- 

 tids and may be homologous, although this is not quite certain. 

 The conditions in Peraiocytwdon suggest that the external 

 talonid cusp of the docodontids is more probably the original 

 pantothere talonid cusp which has here moved relatively exter- 

 nally from its original median position {Amfhithermm) 

 while it moved relatively internally in the dryolestids. Ante- 

 rior to the me"* is a more distinctly separate cusp of about half its height. I n Docodon 

 this anteroexternal cusp looks like the paraconid, but again Peraiocynodon, which 

 differs but little from Amphitherium or Peramus in the trigonid, offers a more prob- 

 able homology. This anteroexternal cusp in Peraiocynodon is just appearing, is, in- 

 deed, hardly visible, and is rather clearly a new structure while the anteroexternal 

 cusp is the paraconid. As might be expected, comparison of the highly specialized 

 Docodon with the differently specialized dryolestids gives different results from com- 

 parison of the more primitive Peraiocynodon with the ancient and most primitive 

 Amfhitherium. 



The me"^ and ^x^ are connected across the middle of the tooth by a stout, oblique 

 transverse ridge. Posterior to this ridge is a basin which constitutes about half of the 

 crown and is clearly a basined talonid. It is open posteriorly or, more correctly, is 

 closed posteriorly by the anterior end of the following molar. 



From near the apex of the anteroexternal cusp (Ppa"^) there runs outward and 

 downward on the anterior face of the crown a cingulum which widens and forms a 

 small, low cusp anterior to the main anterointernal cusp. There are thus seven cusps, 

 three of which have no homologues in the dryolestids." 



Besides the cusps and the main transverse ridge there is a complicated pattern of 

 lesser furrows and ridges. These are not, as might be supposed, random or variable 

 features but are quite invariable throughout the genus, even including the species 

 hitherto placed in distinct genera. From the notch between the pr^ and anteroexternal 

 cusp a furrow runs straight inward to end at the external side of the base of the antero- 

 internal cusp. Another, ending at the same point, runs down the anterointernal face of 

 the pr* from near its apex. Also from near the apex of the pr** a furrow runs straight 

 inward to the notch between the me<i and anterointernal cusp. The talonid is marked 

 " The anterointernal cingulum cusp has an analogue and possible homologue in Peramus, but not 

 in other known pantotheres. 



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