1910.] The Molar Teeth of Peralestes. . 175 



As shown in oblique upper side view (Osborn, 1907, p. 26; this ivork, 

 Fig. 9, A) the molar of Peralestes may conveniently be conceived as a modi- 

 fication of the Didelphis molar type. It is much narrower transversely, 

 the protocone is very high and curved externally, and the large metaconc 

 occupies the postero- external slope of the protocone. The metastyle shear 

 is not so sharp as in Didelphis and there are no small external cingular cusps. 

 The anterior edge of the protocone in the type has been worn by a lower 

 tooth (Osborn). 



The resemblance to a reversed S palacotherium type (Osborn) consists 

 chiefly in the fact that the protocone is flanked by two smaller cusps which 

 are in turn guarded by a heavy basal cingulum. But these resemblances 

 are not sufficient to indicate that the S palacotherium molars fitted into the 

 Peralestes molars. The distorted triangular contour of the Peralestes upper 

 teeth and the asymmetrical interspaces would seem ill adapted to receive 

 the symmetrical triangular contours of the S palacotherium lower teeth. 

 From a study of the relations of the known upper teeth of Peralestes and a 

 comparison with those of its nearest analogue Didelphis, we should be led 

 to expect the following characters in the Peralestes lower molars, which are 

 represented in the accompanying hypothetical reconstruction (Fig. 9.4 ) : 



(1) Ah asymmetrical arrangement of the cusps of the trigonid. 



(2) Protoconid very high, recurved posteriorly. 



(3) Protoconid in horizontal section wedge-shaped, the protoconid- 

 paraconid shear running obliquely forward and dow^nward, the protoconid- 

 metaconid shear directly inward. 



(4) The very large metaconc in Peralestes is even further internal than 

 in Didelphis and must therefore have fitted into the space immediately in 

 front of the trigonid and behind the talonid of the preceding molar. The 

 metaconc may have been received into a distinct fossa on the antero-external 

 base of the protoconid, as in Didelphis and Didelphops. 



(5) Talonid very low, for reception of high protocone above, concave 

 superiorly. 



If this interpretation be at all correct then the lower molars of Peralestes 

 were obviously very different from those of S palacotherium and the removal 

 of Peralestes from the order Triconodonta seems advisable. 



Where then are the allies of Peralestes to be sought ? 



Some of the characters predicated above of the lower teeth of Peralestes 

 are realized in Peravius tenuirostris (Osborn, 1907, p. 27, fig. 16), e. g., the 

 oblique protoconid-paraconid shear, and low upw^ardly-curved talonid. 

 But Peralestes cannot belong directly with Peravius on account of the differ- 

 ent number of molars. Leptocladus dubius (Osborn, 1907, pp. 28-29) agrees 

 with our hypothetical lower dentition in its recurved protocones and six 



