1892.] NATURAL SCIENCES OF PHILADELPHIA. 415 



case of the upper teeth. As before, the initial point must be taken 

 as a simple conical cusp, the protoconid. Most of the existing 

 unguiculates, as well as some recent and many extinct ungulates 

 retain more or fewer teeth which depart but little from this type. 

 In many forms the only addition to the protoconid consists in a 

 small posterior basal cusp, which the analogy of such Mesozoic 

 mammals as Amphilestes and Trieonodon justifies us in regarding 

 as the equivalent of the metaconid of the molars. In many of the 

 Carnivora a second posterior cusp is added, to which, however, it is 

 not necessary to give a name, since it is of little importance and 

 occurs in but few forms. Frequently also an anterior basal cusp, 

 strictly comparable to the paraconid of the molars, is added and a 

 stage like that of the Trieonodon molar is attained, consisting 

 of elements which there is every reason to regard as homologous 

 with the three primary cusps of the molars. There is, however, a 

 great difference as to the regularity with which the para- and metaco- 

 nids are present and in the order of their succession ; one or the 

 other of them may never appear at all, and while the metaconid is 

 more frequently present and generally makes its appearance first, 

 yet this is by no means invariably the case. Another difference 

 from the molars consists in the ultimate fate of the metaconid in 

 the molariform premolar, where it becomes either part or the whole 

 of the talon and always remains on the same antero-posterior line 

 with the protoconid, instead of shifting to the internal or lingual side 

 of the latter. In the premolars, therefore, when a cusp occurs 

 occupying the position taken by the metaconid in the molars, it 

 cannot be regarded as homolos^ous with that element, but rather 

 with the deuterocone of the upper premolar and may consequently 

 be called the deuteroconid. The latter element also varies as to the 

 relative time of its appearance ; sometimes it is the only element 

 present in addition to the protoconid (e. g., Pelycodus, Chriacus, Pro- 

 togonia) or it may be developed after either the para- or the meta- 

 conid, or it may appear last of all, and in very many cases it is 

 altogether absent. Yet when it does appear, its homologies are 

 perfectly obvious. 



A fifth element is sometimes added to the premolar crown, pos- 

 terior to the protoconid, and interior to the metaconid, thus occupy- 

 ing the position held by the entoconid of the molars. Clearly 



