422 PROCEEDINGS OF THE ACADEMY OF [1892. 



CMS, Deltatherium and Tetrachenodon. The later genera of the 

 Proviverridce have exceedingly simple inferior premolars. In Sin- 

 opa i? and * have very obscure metaconids, and in the latter there is 

 an equally obscure paraconid. 



In the Ilycenodontidce p* has only a more or less enlarged meta- 

 conid in addition to the protoconid. In Oxycena the metaconid 

 forms a large trenchant blade ; in Hycenodon the heel is much 

 reduced, but the protoconid is greatly enlarged, so that, with the 

 exception of the canine and m\ p* is the lai-gest tooth in the jaw. 

 Pterodon resembles Hycenodon in this respect. Quercytherivm, 

 clearly a member of this family, is remarkable for the massiveness 

 of its inferior premolars, which almost rival those of Mioclcenus. 



Palceonidis has very similar inferior premolars to those of 

 Oxycena, except for the much greater relative enlargement of p^ 

 In the Bridger genus Patriofelis this enlargement is carried still 

 further and at the same time the number of inferior premolars is 

 reduced to three. 



THdymictis is remarkable for the antero-posterior elongation of 

 p3 and* which have small but very well marked para- and metacon- 

 ids. In p^ the cingulum is raised on the hinder border of the tooth 

 making a second posterior cusp behind the metaconid. 



III. INSECTIVORA. 



In many of the existing insectivores the premolars have undergone 

 considerable modification, but unfortunately we can obtain but little 

 assistance from palaeontology in attempting to make out the steps 

 of this modification. Among American genera of this order there 

 is only one family which is at all well understood, viz., the Leptictidce 

 and in this group, strange to say, the oldest member yet discovered, 

 exhibits the most complicated premolar structure. This genus, 

 Ictop><, has been found only in the AVind River beds at the base of 

 the Bridger, and in the White liiver, though there can be no 

 doubt that it existed in the intervening period. In Idops the pre- 

 molars show an unusual degree of complication for an unguiculate 

 mammal. In the upper jaw J^ is very small and implanted by a 

 single fang. Pj is likewise small but is carried by two fangs, and 

 has a small tritocone, but no deuterocone. In Ps the protocone is 

 very high, and the tritocone, though distinct, is much smaller ; there 

 is also a well developed deuterocone, so that all the molar elements 

 are present, though owing to the disproportionate size of the proto- 



