1892.] 



NATURAL SCIENCES OF PHILADELPHIA. 



417 



/la-' d'' ia'' fux'^ meJ^ An/'- 



Fig. 3. 



Diagram to exhibit the relations of the molar and premolar cusps, showing the 

 fourth premolar and first molar of the left upper jaw and right lower jaw, pr 

 protocone, a^ deutcrocone, tr tritocone, te tetartocone,/« paracone, 7ne metacone, 

 /y hypocone,//-'^ protoconid, /rt<^ paraconid, vied metacoiiid, tad tetarioconid, 

 hyd hypoconid, end entoconid. 



UPPER JAW. 



LOWER JAW. 



Molar. 



Protocone 

 Paracone 

 Metacone 

 Hypocone 



Premolar. 

 = Deuterocone 

 = Protocone 

 = Tritocone 

 = Tetartocone 



Molar. 



Protoconid 



Paraconid 



Metaconid 



Hypoconid 



Entoconid 



Premolar. 



Protoconid 



Paraconid 



Deuterocouid 



Metaconid 



Tetartoconid 



It now remains to consider the facts of premolar evolution as 

 they are exhibited by the various orders of mammals. This is pos- 

 sible as yet for only a small number of groups, the palieontological 

 evidence as to the others being too incomplete. The following 

 orders will therefore be omitted ; Monotremata, Marsupialia, Eden- 

 tata, Tillodontia, Cetacea and Sirenia. In those orders which are 

 taken into consideration, attention will be chiefly directed to the 

 extinct forms. 



I. PRIMATES. 



In this order the Lemuroidea and theMesodonta will be included 

 for the sake of convenience, these groups all belonging to the same 

 general series and characterized by great uniformity in the structure 

 of the teeth. 



The simplest type of premolar dentition in this series is presented 

 by the Puerco genus 3Iixodectes. Of this form only the inferior 

 teeth are known, and the premolars are remarkably simple; they 

 consist merely of a high, acute and recurved protoconid w'ith a 

 rudimentary metaconid as a heel, which can hardly be called a 

 28 



