1892.] 



NATURAL SCIENCES OF PHILADELPHIA. 



427 



proportions are somewhat different ; tliere is a high protoconid, a 

 much smaller deuteroconid and a talon consisting of meta- and 

 tetartoconids ; the cusps are all connected around the periphery of 

 the crown by crests which thus enclose a deep central concavity. 



The teeth of later rodent genera often attain a very high degree 

 of complication and when premolars are present they follow all the 

 stages of molar evolution, at least in the case of the fourth pair, 

 but materials are wanting to enable us to trace the steps of increas- 

 ing complexity. 



VI. CONDYLARTHRA. 



In the family Phenacodontidce some very instructive series may 

 be observed. The superior dentition of Protogonodon {^MiodiBnus 

 pe7itaeus Cope) is unknown, but in the lower jaw, the premolars, 



even the last one, are simple acute 

 cones, but rudiments of a deuteroconid 

 are observable in some specimens, now 

 larger, now smaller. In Protogonia 

 this element becomes enlarged and 

 very distinctly separated from the pro- 

 toconid, and at the same time a rudi- 

 mentary metaconid makes its appear- 

 side, internal aspect. 1 and 2 of ^^^q j^ Phenacodus vortmani, in ad- 



Protogofiodon pentacus, 3 of Pro- . , , . i 



togoma pliciferao prd ^xo\ozox-{xA, dition to these elements, aparaconid 

 pad ^z.\2.com.A, vied metaconid, ^'j' jg developed and the metaconid is ele- 



deuteroconid. (Cope colleciion.) , i t r> • • ] 



^ ^ ' vated. In P. prwiceviis a paraconid 



and a very small tetartoconid are added and p^ is now made more 

 complex by the appearance of the deuteroconid, which is present in 

 rudimentary form, even on \^K A similar series may be made out 

 in the upper jaw. ^ is simple in Protogonia and contains only the 

 protocone, while P* has added the deuterocone. In Phenacodus 

 vortmani, P* has also acquired the tritocone, and besides this we find 

 the antero-intermediate cusp, or conule, analogous to and occupying 



the same relative posi- 

 tion as the protoconule 

 of the molars, though 

 obviously not homolo- 

 ofous with that element. 

 Fig. 7. The occurrence of the 



Upper premolars of Condylarthra, right side. 1, conules is one of esj^ecial 

 p-^ and ^ o^ Phenacodus prinuEvtis. 2, of P.vorl- . ^ A' f 



ma7ii, 3, p* of Protogonia siibquadrata, pr, proto- interest, as tCUamg tO 

 cone, d, deuterocone, tr, tritocone. (After Cope). show that the premolar 



Fig. 6. 

 Fourth lower premolar of right 



