414 PROCEEDINGS OP THE ACADEMY OF [1892. 



many Insectivora and in some forms of Artiodactyla (e. g. Dicoty- 

 les.) In many forms this pattern of tooth does not occur in advance 

 of the fourth premolar, while in others the second and third grad- 

 ually assume the same structure, as in most of the Eocene perisso- 

 dactyles. Even in Phenacodus F* is trigonodont, while P^ consists of 

 the protocone only. 



The final step in the conversion of the premolars to the molar 

 pattern is given by the addition of a fourth main element at the 

 postero-internal angle of the crown, the tetartocone, which thus 

 corresponds in position to the hypocone of the molars. The incipi- 

 ent stages of this element maybe observed in Helaletes (Desmato- 

 therium) among the perissodactyls and in Agriochcerus among the 

 artiodactyls, where it occurs as a minute and variable cusp just 

 posterior to and hardly separated from the deuterocone. 



So far as I have been able to observe, the scheme of development 



of the premolar crown is quite constant, 

 and for p^ universally so, and the nomen- 

 clature which is proposed for the pre- 

 /^^^^^5^ %v^^>^ molar cusps is intended to express their 

 ^ ^VBP^ \^ order of succession, as they appear in 

 Fig. I. P*. However, exceptions to this order of 



Procamdus occidentalis. succession do occur in the anterior pre- 

 Third and fourth upper pre" molars. For example, in F^ of Procame- 



molars of left side, «/ deutero- , ,, • . ~u' x. • ^4 : 



cone; x additional anterior ^««. the inner crescent, which in P_1S 

 cusp. (After Leidy.) formed by the extension of the deutero- 



cone in both directions, is produced by crests from the anterior 

 and posterior edges of the protocone, which have not yet come into 

 contact with each other and thus leave a gap opposite the apex of the 

 protocone. As we shall see later, the posterior ridge in this tooth 

 probably represents the deuterocone. Again, in Hyracotherium 

 venticolum v" consists of two cusps, the proto- and tritocones, which 

 are in the same antero-posterior line and the deuterocone is little or 

 not at all developed. Other exceptions might be cited. 



The development of the inferior premolars appears to be some- 

 what less regular and constant than 

 that of the superior. As in the upper 

 jaw, the complication begins with j? and 

 ^^^- ^- advances anteriorly, but it is worthy of 



Hyracotherium venticolum. ^Q^jce that in many forms the complica- 



Second upper premolar of left ^ j . ^ j premolars begins 



side, /r. protocone; /;•. tritocone. ''"^" ua u iv. ^ I ^ o 



(After Cope.) earlier and proceeds farther than in the 



