1892.] NATURAL SCIENCES OF PHILADELPHIA. 421 



but the protoconid is relatively reduced, and the metaconid 

 enlarged. In Mesonyx the paraconid makes its appearance on p^ 

 No member of this family develops a deutero- or tetartoconid in 

 the inferior premolars. In the Mesonychidce, the lower premolars 

 thus become molariform, bat the resemblance between the two 

 classes of teeth is due not only to the complication of the premolars 

 but also to the simplification of the molars, in which the paraconid 

 is reduced and the meta- and entoconids have altogether dis- 

 appeared. The elements composing the crowns of the two categories 

 of teeth are more nearly homologous than is usually the case, the 

 proto- and paraconids being common to both, but the talons are not 

 homologous, being formed in the premolars by the metaconid, and 

 in the molars by the hypoconid. 



In the Triisodontidie p^ has a distinct resemblance to the corre- 

 sponding tooth in the Mesonychidif, but the paraconid is not devel- 

 oped, and in Triisodon and Sircothraudes the lai'ge, transversely 

 directed heel is divided into two parts, the meta- and tetartoconids. 

 In Goniacodon the latter element does not occur. 



In Ardocyon and Clcenodon p* is remarkable for its great relative 

 size, but is very simply constructed, having in addition to the 

 protoconid merely an obscurely developed metaconid. In Tetra- 

 clcEiiodon, however, p*" has all the elements of a molar, but the 

 paraconid is very small, and the talon neither so high nor so complex 

 as in the molars. 



Oiriacus (if we may assign that genus to the Creodonta) presents 

 an entirely different type of p^ from any which has been yet 

 described in this group, and one which is much more characteristi- 

 cally mesodont than creodont. In this tooth the only element 

 besides the protoconid which is present, is a small but perfectly 

 distinct deuteroconid upon the lingual side of the crown. Epichria- 

 cus has in addition to this a very small paraconid. In Deltatherium 

 the " primitive triangle " (a term which is applicable to the premolars 

 only by analogy, as it is rarely attained and then only in 

 advanced stages of differentiation) is almost as completely developed 

 as in the molars. In addition, there is a rudimentary heel in the 

 form of a very small metaconid, which, strange to say, has not kept 

 pace with the development of the deuterocone of P3. Not including 

 the Me-<OJiychidce, which belong in a somewhat different category, 

 there are not less than three genera of creodonts, in which 

 attains more or less completely to the molar pattern, viz., Epichria- 



